This collapses runs of whitespace into single spaces and removes leading and trailing space from each line.
Why irregular spacing accumulates
Copying between applications introduces non-breaking spaces, tabs and double spaces. None are visible, and all of them affect comparison.
The invisible failure
Two entries that look identical but differ by a trailing space will never match. Deduplication misses them, lookups fail, and nothing in the interface explains why.
Normalise before comparing
Any operation involving matching — deduplication, set operations, joins — should be preceded by whitespace normalisation, or its results are unreliable.
Try it: Normalize Whitespace on SeoWolf's Notepad.