Range interval insert which should split into unique ranges
Japt, 44 bytes

rò
íU c ò@Vc øX ©X¥YÃke@V®ròÃc øXÃ®gJõ0ÃcV n


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Reworked it, at the cost of 1 byte.
Now handles the [[2,2]],[1,3] test case properly.

Explanation:

rò                                           #Turn the second input into a range
íU c                                         #Duplicate each item in that range
     ò@      ©X¥YÃ                           #Split in the middle of the duplicates that:
       Vc øX                                 # Appear in the first input
                  k                          #Remove the segments where:
                   e@       øXÃ              # Every item in the segment is in...
                     V®ròÃc                  # A range from the first input
                               ®    Ã        #For the remaining segments:
                                gJõ0         # Get the first and last items
                                     cV      #Add those pairs to the first input
                                        n    #Sort the whole thing


Worked Example:

Input: [[2,2],[5,7]],[1,8]

Second input as a range: [1,2,3,4,5,6,7,8]

Duplicate each item: [1,1,2,2,3,3,4,4,5,5,6,6,7,7,8,8]

Numbers that appear in the first input: [2,5,7]

After splitting: [[1,1,2],[2,3,3,4,4,5],[5,6,6,7],[7,8,8]]

Numbers within the first input's ranges: [2,5,6,7]

Segments using only those numbers:[[5,6,6,7]]

Remaining segments after those are removed: [[1,1,2],[2,3,3,4,4,5],[7,8,8]]

First and last item of each: [[1,2],[2,5],[7,8]]

Merge with the first input: [[1,2],[2,5],[7,8],[2,2],[5,7]]

Sort it: [[1,2],[2,2],[2,5],[5,7],[7,8]]

Which matches the actual output of the code