Have you searched for similar requests?
Yes

Is your feature request related to a problem? Please describe.
When importing characters in different formats, it does not duplicate checks on them even if they are a perfect match.
The issue is when you work on a large collection of characters its quite easy to get dupes.
Its even easier to get dupes if you download characters from multiple sites!

Describe the solution you'd like
If character names matches then compare each field of a character and skip the import if it all matches.
This way you will end up with less duplicates even if you do large imports from fresh downloads.

Describe alternatives you've considered
Alternative is to compare the hash value of said file, maybe SHA256? Since the image or json is all conistant with the content then each update of said character will have a different hash value.