Any two in a row is surprisingly likely, it’s just the chance of pulling a specific legendary (similar to how the chance of rolling a die and getting the same number twice in sequence is just 1:6).
My gut feeling says that duplicate legendaries might possibly show up too often. My brain says that gut feelings in regards to probabilities are almost always incorrect, only data is worth looking at. 
There are 108 legendaries +1 imp currently available.
If you pull 2 of the same kind in a row, the odds are “just” 1:108.
Even the probability for 1000 in a row are greater then 0. And that at least someone will achieve such an odd outcome is way higher.
We have also to consider that you might pull 3 legendarys where 2 are of the same kind might look like 2 in a row but could also be A-B-A but sorted alphabetically by the game.
But in the end this doens’t lead to anything since we don’t know the “real distribution” from the devs. Maybe there is a trigger which turns on once it starts to rain or whatever 