I ran FASTQC on a human gut metagenome sample and found that I have a duplication rate of about 80%  Does this seem too high? I checked out some environmental samples and saw approximately the same rates.

I've read papers that recommend de-duplicating reads before analysis because they're most likely PCR artefacts. But I've read papers that recommend keeping all reads since some high abundance species will be sequenced deeply and some reads may be seen more than once. Any thoughts on the matter?
High read depth on small templates or in your case low-diversity metagenomes does indeed lead to high duplicate rates (see your answer to Josh Herr above). We see this on amplicon datasets all the time.

For your analysis I would attempt analysis - if mapping reads - with and without the duplicates. Remember to add a Mapping Quality filter afterwards too. There will be information in both analysis methods. Remember to check which parts of the genome duplicates are coming from.

Typically, in our pipeline we remove duplicate and low-quality reads before analysis with several tools (mapping to human + bacterial genomes) and after mapping (eg. with Picard).