I frequently hear this.

Problem:
The adverb “back” in “reply back” is redundant.

Explanation:
The “re” in “reply” means back. Therefore, one could argue that a “reply back” would never reach the intended recipient.

Solution:
“I’m waiting for him to reply.”

I frequently hear this.

Problem:
The adverb “back” in “respond back” is redundant.

Explanation:
The “re” in “respond” means back. Therefore, one could argue that a “response back” would never reach the intended recipient.

Solution:
“Please respond.”