Add the ability to define duplicate global song pointers

[KungFuFurby]

Heard about this on the SMWCentral Discord server when NBTD/No Body The Dragon brought it up when they were expecting this situation, but instead got the same song inserted twice, wasting space. The mechanism would be pretty simple: just be on the lookout for a duplicate filepath in the global songs section, then take advantage of the situation accordingly.

#60 also has some relevance to this, because this would make defining duplicate songs easier over there as well in the groups.