Does it change the order of my lines?
No. Each line stays where it first appeared and later repeats are removed. If you want the list sorted as well, use the Sort lines tool afterwards.
Are “Apple” and “apple” duplicates?
By default, yes. Turn on “Case-sensitive” to treat lines that differ only in capitalisation as different.
What does “Ignore surrounding spaces” do?
It trims spaces and tabs from the start and end of each line before comparing, so “linus@example.com” and “ linus@example.com” count as the same line. The trimmed version is what you get back.
Is there a size limit?
Only your device’s memory. Lists with hundreds of thousands of lines work fine because nothing is uploaded.