How can I open multiple instances of the same document in Evince?

Open the file then from menu:

File -> Open a Copy

I've voted for CYrus's answer, which I think is probably best for most users. If you really want to do it from the command line, I suppose there's always a script like this:

#!/bin/bash
PDFNAME="$1"
TEMPNAME="$(mktemp --suffix=.pdf)"
cp "$PDFNAME" "$TEMPNAME"
evince "$TEMPNAME"
rm "$TEMPNAME"