Use distinct(ListOfVars) to specify which attributes you 
want to be distinct:
test_distinct :-
  findall(UserName,
          {[],
           se_lt_x :: [a-UserName,
                       c-Key],
           Key >= 7,
           distinct([UserName])},
          L),
  length(L, N),
  format('~w solutions~n', [N]).
CALL  : user:test_distinct/0
26 solutions
EXIT  : user:test_distinct/0 (0.098133s, 0.00cpu, 1,488 inferences)