Compare two lists
Paste the two lists in any order and see straight away what is only in the first one, only in the second one and in both, with duplicates counted and three lists to copy.
Your lists stay in your browser. Nothing is uploaded anywhere and nobody else sees them.
How to use it
Paste one list in the first box and the other one in the second box, choose how the entries are separated (usually one per line, but commas, semicolons and tabs work too) and press Compare. You get three ready to copy lists, what is only in the first one, what is only in the second one and what is in both. The order does not matter and you do not need to sort anything first.
Why this is not a line by line diff
Text comparison tools line up rows by their position, so moving one entry from the top to the bottom shows up as a removal plus an addition. Here every entry goes into a set (the browser Set structure) and the only question is whether it also appears in the other set. The cost grows linearly with the number of entries, so two lists with hundreds of thousands of rows are compared instantly, while a positional diff on those sizes grinds to a halt.
Case, spaces and accents
The three checkboxes never change your text, they only change the key used for the comparison. With case ignored, SMITH and smith become the same entry; with spaces ignored the ones at the ends and the double ones in the middle disappear, the kind you get when pasting from a PDF or an email; with accents ignored Peru and Perú match, handy for surnames but risky when the accent really changes the word. The result always shows your original text, and when the two spellings differ the last list tells you.
Duplicates inside one list
A set has no duplicates, so an entry repeated five times in the first list appears once in the result. To keep that information the summary says how many entries were read, how many are different from each other and how many are repeated. When those numbers disagree you have duplicates to clean up, and the Remove duplicate lines tool does exactly that.
Stated limits
Everything happens inside your browser, the lists are never uploaded anywhere. The match is exact, not fuzzy, so John Smith and J. Smith stay two different entries and no spell checking is involved. The tool reads plain entries, not CSV columns, so if you paste rows with several fields and pick the comma, every field becomes an entry of its own. At most 500 entries per list are drawn on screen to keep phones responsive, but the copy button copies all of them.