Yes these stack, but keep in mind these are not identical Levitate buffs. The player-cast Levitate spell only works outdoors. By feature, or by bug, the Pegasus Cloak allows indoor Levitation. And before anyone suggest the Cloak levitate should be considered a "better" spell and overwrite the outdoor only Levitate, I think the duration from the cloak is different (shorter) as well, so it may not be quite that simple.
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