feature request: don't import duplicates

[pkoevesdi]

When importing public holidays, Kimai imports duplicates after second import. Would be nice, if it skips already existing entries.

Use case: in our company we give the combined public holidays of two bundesländer for all workers. With this feature, we could import from both bundesländer into the same holiday group and have the combination of them, instead of duplicates for all holidays that exist in both bundesländer.