In Solr, I am using the MoreLikeThis handler. I am passing in the unique id of a document that already exists in the index in order to find related documents. Does specifying mlt.mindf=1 include the previously mentioned document? If I want to be sure it exists in at least one document other than the one I pass in, should I set the value of mlt.mindf=2 instead?
That's a fun project your'e doing :D
And your'e correct. You should set the value to 2 if you want to make sure that there is at-least one other document with the same term.
Tip:
If you're automatically finding related documents,i.e, the relation(term) is not specified by a user, make sure you search for good keywords to find related documents by filtering out unwanted words(use any of the available filters to do this).
You could also suggest some words(maybe 5,6,..) and use javascript to make something like the Wonder-wheel.
Have fun :)