sort -u file1 file2 gives the union. cat file1 file2 | awk '!seen[$0]++' gives it while preserving first-seen order. The operation is trivial; the correctness lives entirely in what you do beforehand.
Equality is a decision
Set operations compare for exact equality. Running Shoes, running shoes and running shoes are three distinct members. Every set operation over un-normalised marketing data overstates the result, and the overstatement is invisible.
The normalisation that usually matters
Trim whitespace, collapse internal runs of spaces, case-fold, and strip a trailing slash if the values are URLs. Apply the identical pipeline to both inputs — normalising one side only is worse than normalising neither, because it guarantees no matches at all.
Unicode equivalence
Two visually identical strings can differ in bytes when accented characters use combining marks in one source and precomposed forms in the other. Normalise to NFC before comparing anything sourced from different systems.
Try it: Merge Two Lists (Union) on SeoWolf's Notepad.