Avoid reporting duplicate errors

[bryanrsmith]

In the current implementation there are two forms of violations that can cause redundant errors.

Members appearing multiple positions late

Given an expected order of [ a, b, c ], and an actual order of [ b, c, a ], the rule will currently report 2 errors:

1. expected a to come before b
2. expected a to come before c

The second error is redundant and should not be reported.

Members appearing multiple positions early

Given an expected order of [ a, b, c ], and an actual order of [ c, a, b ], the rule will currently report 2 errors:

1. expected a to come before c
2. expected b to come before c

This can be more succinctly reported as a single error: "expected c to come after b".