get_the_terms returns every term twice

Wherever I call get_the_terms (or as a following problem get_the_terms_list) in my theme, the returned array contains every term twice.

I could only suppress this by using clean_post_cache before getting the terms.

All entries in DB are correct: Calling the queries in the DB directly returns the terms only once.
All cache Plugins have been disabled
Disabling all plugins did not solve the problem.

Has anyone encountered the same problem and might give me a push into the right direction?