How to Keep Track of Twitter Replies

I am starting to use Twitter to find potential leads. My strategy is to search for tweets where a customer is asking for help and reply to those tweets with a link to a free tool I made.

The issue is that some customers tweet for help multiple times a month, and I don't want to reply to them more than once a month. I am unable to figure out an easy way to track and filter out who I have replied to in the last month. Checking manually against a list is starting to get cumbersome.

Any ideas / little hacks on how to solve this? Any products out there that help with this?

Thanks.