How bad is duplicate content for SEO?

Hey Guys,
So I have some affiliate dating sites running - by default, they use robots.txt to block google from looking at the members pages - and as such, google is only indexing about 6 of my sites pages.
I've looked at my competitors and Im owning them in every other aspect (incoming links / domain age / etc / etc) - my only "weak point" is my 6 indexed pages vs their several hundreds.

Now, should I clear up the robots.txt so that it indexes the members and gives my site more strength or would I be shooting myself in the foot because of duplicate content?

This is something I've been dealing with for years. I used to block Google (through robots.txt file) from viewing my member pages too and about a year ago, I opened it up to allow Google to index everything. My ranking really didn't change much, but it went up a little. My advice would be to allow allow Google to index everything and see what happens. You can always put it back in if you don't like the results.

All websites have duplicate content...its built into the variability of URLs. Google's solution to this is to rank the highest PR URL for the content unless there is another stronger signal like authorship or canonical URLs that are contrary.

I would think a dating site would create liabilities by having member profiles public. Am I wrong?

Allow Google to view all of your pages but at the same time, try to make your pages different in some way by adding some unique text variations.