Zolmitriptan Dimer Impurity,(4S,4′S)-4,4′-[[4-(Dimethylamino)butane-1,1-diyl]bis[[3-[2-(dimethylamino)ethyl]-1H-indole-2,5-diyl]methylene]]bis(1,3-oxazolidin-2-one); 2,2′-[4-(Dimethylamino)butane-1,1-diyl]bis{5-[(S)-(2-oxooxazolidin-4-yl)methyl]-3-(2-dimethylaminoethyl)indole},(4S,4`S)-4,4`-[[4-(Dimethylamino)butane-1,1-diyl]bis[[3-[2-(dimethylamino)ethyl]-1H-indole-2,5-diyl]methylene]]bis(1,3-oxazolidin-2-one)