Updated sp3 dupe check \w has_duplicates property

[bmatv]

As has_duplicates is an already a property inside the sp3 dataframe it is literally free. You may argue that index.duplicated() before if is not a waste as you still need it to compute first dupe, but I could speculate that 99% of sp3 files come without dupes.

[treefern]

Nice improvement :) This is also a bit more readable.

[bmatv]

@treefern Originally I added a comment on has_duplicates to you branch but never clicked on Finish your review button so it was a pending [read as invisible] comment. But this branch is also a great test on how-to commit/merge etc when one is a poor contributor, not a maintainer :)