Warn of duplicate entries

[rhagenson]

In all input formats (relatedness, demographics, and pedigree), I should add warnings if duplicate entries are found.

For relatedness (currently --input), this would be cases of:

1. ID1 and ID2 repeated with different Rel entry
2. ID1 and ID2 switched with different Rel entry

For parentage (currently --parentage), this would be cases of:

1. ID repeated with different Sire and/or Dam
2. ID repeated with switched Sire and Dam

For demographics (currently --demographics), this would be cases of:

1. ID repeated with different Sex and/or Birth Year

For all of these, if it is simply a duplicate entry it should be downgraded to an information message rather than a warning because there is mostly no issue with duplicates, but a user might want to know.