This sucks! I've actually submitted a Pull Request to fix this issue, so that hopefully in future this won't be a problem.
If you're using source control, one person will be writing the .sud file and
probably importing it into a .uasset as well, and adding both to source control.
However, when someone else gets both these files from source control, it's
possible that UE will prompt them to re-import the .sud file, thinking it has
changed - even though it's the same one that the other person used to import the
asset.
Unfortunately this is a general problem with how Unreal detects changes in source files for re-importing when the source is also store in source control.
The official UE docs cover this scenario, and advise to turn off the editor setting "Detect Changes on Restart" (actually this is now called "Detect Changes on Startup"):
Basically, UE can't tell that the source file on disk is the same one used to create the asset that's been imported by someone else. UE caches file hashes to avoid imports, but it only does so for things it's imported itself at some point in the past. If someone else did it, the hash won't be in the cache and it'll just go by the fact that the timestamp is later than the last it knew about. I've picked over the asset reimport code and can't see any way of making it use a hash in the uasset to compare, the custom asset import code doesn't seem to have any access to the decision about whether to prompt to reimport.
Changing this setting will fix the problem, but has the side effect of making UE only prompt you to re-import assets that you update while the editor is running. If you're in the habit of changing source files then starting UE, you'll no longer get the auto-import prompt for this.
Because SUDS assets are entirely imported from the text .sud file, if you have
merge conflicts you only have to resolve them in the .sud file itself, then
reimport the asset using that resolved version.