I've seen on old thread (https://community.rws.com/developers-more/developers/language-developers/f/sdk_qa/1041/re-import-imported-package) which has not had any activity for ~7 years, yet the original post was exactly the same as the problem I am experiencing. Let's give it another try!
My scenario is I have 46 packages x 4 languages which were successfully generated via an API routine.
Once translated, all packages were successfully imported using another API routine.
13K+ x 4 files were successfully generated/exported using another API routine.
Subsequent analysis of the resulting build of the translations meant we had to have corrections performed on the original packages, which were then duly returned.
Now I find I cannot import these packages via the API because the API throws an InvalidOperationException exception if the package is already imported.
The GUI shows a pop allowing me to "force" import.
The InvalidOperationException method needs an overload allowing you to force the import just like the GUI allows you to do.