If EQUALS is in effect, DFSORT will keep the first record. If NOEQUALS is in effect, DFSORT can keep either record. You can specify EQUALS like this:
//DFSPARM DD *
You can use the SELECT operator of DFSORT's ICETOOL with the FIRST or LAST operand to tell DFSORT to keep the first or last record, respectively.
If you're not familiar with DFSORT and DFSORT's ICETOOL, I'd suggest reading through "z/OS DFSORT: Getting Started". It's an excellent tutorial, with lots of examples, that will show you how to use DFSORT, DFSORT's ICETOOL and DFSORT Symbols. You can access it online, along with all of the other DFSORT books, from: