The reason I ask is I have 1 site on adwords and want to put up another with a slight overlap in some content (maybe 20% worth). Could the site/s be slapped with a low QS due to repeating some content?
Do you mean 20% of the keywords are duplicated? If that's what you mean then the Quality Score will stay the same but you will outbid yourself on the duplicate keywords.
what i've experienced is that some duplicate content did not affect my QS. i had multiple landing pages with the same offer on top and varied content below the form submission for my various ad groups. i put the related ad group keywords in the page titles, headers and page content, but these pages were blocked from being crawled by google, so all traffic was via PPC. QS was outstanding. it's really all about testing. your ad copy has to be good too to get a good CTR or else QS will still go down.