When you're writing a song, how do you know it isn't just a song you've heard before but don't recognise?

Not sure how to help you out with it, but you're at least not alone. Robert Smith from The Cure had the same problem with the song "Friday I'm in Love": https://en.m.wikipedia.org/wiki/Friday_I%27m_in_Love
"During the writing process, Robert Smith became convinced that he had inadvertently stolen the chord progression from somewhere, and this led him to a state of paranoia where he called everyone he could think of and played the song for them, asking if they had heard it before. None of them had, and Smith realised that the melody was indeed his."
- Similar story with Yesterday by the Beatles. Paul McCartney was convinced he had unconsciously plagiarized the song after he’d supposedly heard it in a dream.
  
  Is that what inspired the movie?
- The same thing happened to John Anderson when he was writing Seminole Wind

No, you see it's: "dun-dun-dun-dudu-dun-dun dudu dun-dun-dun-dudu-dun-dun"
not
"dun-dun-dun-dudu-dun-dun, dun-dun-dun-dudu-dun-dun"
- Stop...
  
  collaborate and listen?
  
  Hammer time?
- No joke but I remember many years ago seeing vanilla ice basically do that to explain why ice ice baby is nothing like under pressure
  
  Ice counts the dings “not the same”

Nope, happens a lot to me, too. Worst part is that whatever you're accidentally plagiarizing, will immediately sound great and will be really easy to write, because of course, you've listened to it before. And it can be nigh impossible to distinguish between accidental plagiarism and just being in a flow.

Everything is derivative of something else. Thag made that drumbeat on a rock 20000 years ago and it has passed down in oral history to eventually be in a Nirvana song.
This sometimes results in songs like Dani California that are almost certainly overt or unintentional copies of another song. When you find out your song is subjectively too close to another song you do the right thing, whatever that may be between you and the original musician.
- Crug hit rok fist.

Art is theft
All art is inspired by other art, it grows, evolves, eats itself, parodies life, informs living.
I wouldn't worry about it
- https://www.youtube.com/watch?v=jcvd5JZkUXY
  
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  https://www.piped.video/watch?v=jcvd5JZkUXY
  Piped is a privacy-respecting open-source alternative frontend to YouTube.
  I'm open-source; check me out at GitHub.

Don't worry about it. There are only so many progressions. Everything else is just variations within them, with bass lines, melodies and rhythms.

I simply don't think about it. If Chino Moreno can get away with using the chorus chord progression of "Hit Me Baby" in one of his songs and the most that happens is laughter about it in the comment section, it's not an issue.
Alternatively, there is nothing new under the sun. Music has seven fundamental notes that can only be arranged so many different ways and still sound pleasing to the ear. It's an inevitability that somewhere, you're going to use the same chord progressions as thousands if not hundreds of thousands of other composers, you're gonna write the same licks, you're gonna play the same riffs. The difference is in atmosphere, genre, and performance. Don't stress it too much.
- Deftones have so much staying power, like they keep making good stuff after 30+ years now. I know Koi no Yokan is like over a decade old, but it's so good, and that was wayyy after their popularity peak.

Run it through shazam (or a foss alternative if there is one) and check?

You don't. I used to write music, and I would frequently think I'm writing a melody only for it to turn out to be something I heard in the background of a TV show or something.

I've done this. Yes, they did exist. That's one of the risks of creating songs from melodies stuck in your head
- What was the song?

No you see, that's the secret. All my songs are just me Weird Al-ing every aspect of them.
Eventually they're different enough that they're truly mine.

Copyright bullshit has made artists paranoid.

Why worry when you are just writing a song ?
Write the song about the song sounding like another song.
- This is just a tribute
  
  a tribute song is still a song

It might be similar to a song you've heard but you're misremembering the notes of the existing song.
Maybe try playing it for an app that recognizes the song that's playing and then listen to any songs it guesses might be the song.

It's normal.
Especially if you're writing in a popular style, tons of musicians just use the same chord progressions over and over .
Once you start layering things on the melody, it's pretty unlikely that it will resemble anything too closely.
As long long as you're just using your own creativity, I think it's very unlikely you'll just clone a song.
If you're really worried about it, you can just change a few notes in the melody on the page in a way that you wouldn't think to sing "naturally"

Reminder Deadmau5 has accidentally remade Sandstorm multiple times.

It is a problem for other people too, but I would argue it's a very small insignificant one. Unless you're ripping off an entire song and it's not parody, you're fine.

You don't lol. There have been instances in the past when pro musicians did this

Nothing is ever truly original, everything you create is a remix of things you encountered after they are processed by your subconscious. And that’s ok. Even if your song will end up to be very similar to another one it will be your own spin on the musical “idea”. Go for it

Related: Will We Ever Run Out Of New Music - Vsauce
- Here is an alternative Piped link(s):
  Will We Ever Run Out Of New Music - Vsauce
  Piped is a privacy-respecting open-source alternative frontend to YouTube.
  I'm open-source; check me out at GitHub.

I'm not a song writer but it seems to me a lots of songs can share some similar chord progression without being in any way the same. It can be more or less obvious.
I feel like, as we're immersed into music, when creating music what we hear in our head can and will be influenced. It probably should be too.
Because even so, you have more than one influence, you don't put them like anyone else and that's where you start putting something that's you, into it.
But to me that also mean what you feel is not only normal for a song writer, but also to any creative process.
I myself got quite obsessed at some point with this question of what is "original", what is creation.
It's pretty philosophical though, on a more practical point of view the best solution is to be learn to recognize your influences in general, and start to build your own style from them. Then you'll know even if one melody resembled another it's still your song. That takes a good level of expertise to define yourself though, and is never really fixed, wich will mean the question can come back often.

You don't. It happens. There are only so many notes. As Picasso said, "good artists borrow, great artists steal"

You’ll constantly be influenced by what you listen to. If the rhythm and everything really feels that way I’d probably be humming it to Siri or google to have it find the song.

You can't, so you must assume that whatever you create has already been created and if people notice it's a sign you did well to emulate the source material.

When I come up with something particularly good, I take a screenshot of the melody + chord symbols and send it to a couple of my musician friends to ask if they recognize it as anything.
One time, they identified an idea as the bridge to an old pop song, but otherwise I've thankfully been in the clear.

You don't have that problem when you write outre conceptual pieces.

Sing the memory into some sort of AI music finder and see if it finds anything the same. If it doesn't, or you're sure you've never heard the song it produces, your tune is probably genuinely yours.

Same here. A few song ideas pop into my head, and then weeks later I realize that I'm basically unknowingly stealing a melody from someone else.
This could be a problem once I start my music career. Because copyright.

I think I heard that there's software to help you find similar songs that some pros use, but it's probably only something that really big companies need to worry about.