It's not needed. In this sentence "but" and "though" mean essentially the same thing. You would say, You do not have to do it again, though. OR ..., but you do not have to do it again. You may hear this in casual conversation, but using equivalent words in the same sentence is called ""redundancy"and should be avoided in your writing. Another example of redundancy: "I got up at 7:00 AM this morning". This is a common expression, but you don't need to say "morning." It's redundant because AM means morning. You should say "7:00 this morning" or "7:00 AM" Please let me know if you further questions.

You can remove the word 'though' and the sentence will still make sense and be correct.