question about 4sum problem (python code)

[feng-1985]

How to understand this line of code?
while l < r and nums[l] == nums[l - 1]:
l += 1
while l < r and nums[r] == nums[r + 1]:
r -= 1
In 2sum problem, we always do this:
while l < r and nums[l] == nums[l + 1]:
l += 1
while l < r and nums[r] == nums[r - 1]:
r -= 1