I saw these two words the other day and wondered whether their definitions are identical.

Problem:
These two nouns are not quite synonyms.

Explanation:
The noun “respondent” means a person who responds.

The noun “responder” means a person or thing that responds.

So a respondent can be a responder, but a responder is not necessarily a respondent.

Solution:
Use “responder” as the generic noun. Use “respondent” when referring to a person.