Can I mash the same song with itself?

No. The two sides have to be different tracks.

2 credits per request, plus 1 credit when the lyrics are written for you. A failed request is refunded.

Can I use an uploaded file as a source?

Yes. Import the audio first and it becomes selectable like any library track.

Good to know

Mashup takes two sources and produces a single new track. You choose both sides, the direction of the blend, and whether the result keeps vocals.

Primary and secondary source: Two different tracks. The same song cannot be used on both sides.
Style: The direction that holds the two sources together.
Lyrics and prompt: What the combined track should say. You can copy the lyrics of either source into the field instead of writing new ones.
Instrumental: Keeps the blend free of vocals.
AI lyrics writer: Writes a new lyric for the blend: 1 credit per request, refunded if it fails.

Pick two different tracks, one for each side.
Describe the blend and decide whether to use the lyrics of one source or write new ones.
Create the combined track for 2 credits.

Both inform the new track; neither is simply pasted over the other, and both originals stay untouched in your library.