Deal with duplicate tyvars in type declarations

compiler/GHC/Tc/Gen/HsType.hs

testsuite/tests/saks/should_compile/saks018.hs

testsuite/tests/saks/should_compile/saks021.hs

testsuite/tests/saks/should_fail/all.T

testsuite/tests/saks/should_fail/saks018-fail.hs

testsuite/tests/saks/should_fail/saks018-fail.stderr

testsuite/tests/saks/should_fail/saks021-fail.hs

testsuite/tests/saks/should_fail/saks021-fail.stderr

testsuite/tests/typecheck/should_compile/T24470b.hs

testsuite/tests/vdq-rta/should_fail/T24604.hs

testsuite/tests/vdq-rta/should_fail/T24604.stderr

testsuite/tests/vdq-rta/should_fail/T24604a.hs

testsuite/tests/vdq-rta/should_fail/T24604a.stderr

testsuite/tests/vdq-rta/should_fail/all.T