Keyword cannibalization happens when two or more of your own pages target the same search term. Instead of one strong page, Google sees several weaker ones competing with each other, and rankings, clicks and links get split between them.
Paste your keyword map one pair per line in the format "keyword | url", exported from a rank tracker, a spreadsheet or your content plan. The checker groups every keyword, flags the ones claimed by two or more different URLs, and counts how many keywords are clean.
Run the check after every content audit or site migration so small overlaps never grow into lost rankings.