Since 1955, ALL antiphons are doubled even on ferias, this happened when feasts were given the rank of either 1ST Class, 2ND Class, etc, getting rid of the mush older medieval way of feasts that were Doubles, semi-doubles, etc.. Its a pity it was changed really, as it showed a gradation of the solemnity of a feast in a rather subtle way (i.e. NOT singing the antiphon entirely before the psalms only at the end), but it does make everything much simpler as now you just sing the whole antiphon ALWAYS. Don't be fooled by later editions of the LIBER USUALIS they never changed the text, probably as it would be too expensive, but do give the new rubric in the later editions somewhere in the beginning I think and nowhere near the actual office texts.