There is a real difference between repurposing an idea and cross-posting a file, and audiences detect the second one instantly.
What gives it away
Watermarks from another platform. Aspect ratios that do not fit. Captions referencing features the current platform does not have. Hashtag blocks in places that do not use them. Calls to action pointing somewhere else.
Each one signals that the post was not made for the person reading it, which is exactly the impression you cannot afford.
Repurpose the idea, rebuild the artefact
Take the core insight and rebuild it in the native format. A long video becomes a short with a different hook. A carousel becomes a text post with the argument compressed. A blog section becomes a two-minute talking-head clip.
This is more work than re-uploading, and considerably less work than having a fresh idea for every platform.
The tag layer changes too
Tag counts, vocabulary and conventions differ per platform. The idea transfers; the tag set does not. Regenerate it for each destination — the same keyword with a different platform selected produces a genuinely different set.
A workable cadence
One substantial idea per week, rebuilt for two or three platforms, beats seven thin posts spread across five. Fewer ideas, better executed, in the right format for each place.