#4862 solve doesn't deduplicate subscripted variables correctly

[David Scherfgen]

Spotted by Claude:

(%i1) solve(p[1] = 5, [p[1], p[1]]);
(%o1) [[p[1] = %r160,p[1] = 5-%r160]]

(%i2) solve(p = 5, [p, p]);
(%o2) [p = 5]