Research arrives from everywhere: a keyword tool, Search Console, the sales team's list of terms customers actually say, competitor page titles, and the client's own spreadsheet. Five lists, heavy overlap, no single view.
Merging them properly
Union them one pair at a time. The result is every distinct term anyone contributed, with the overlap collapsed rather than counted twice.
Normalise before you merge
Case and trailing spaces differ between sources, and unmerged variants inflate the total. Lowercase and trim everything first, or the union reports more distinct terms than genuinely exist.
Keep a record of origin
Before merging, note the count from each source. When the union comes to 5,000 from five lists totalling 12,000, the overlap is the interesting finding — heavy overlap means your sources agree and the research is probably complete; almost none means someone is looking at a different market than everyone else.
Try it: Merge Two Lists (Union) on SeoWolf's Notepad.