Free tool
Portfolio x-ray
Most portfolios hold the same companies several times over, through funds that look unrelated. Paste what you own and the x-ray reports every pair that duplicates itself, ranked by how much.
Up to 20 positions and 25 runs a day on the free version. Runs in your browser; nothing you type leaves the page.
What the x-ray checks
For every pair of funds you list, it computes how much of their combined weight sits in companies both hold, using the issuers' published holdings. Pairs above 60% are flagged, because at that level the second fund is mostly repeating the first.
Limits worth knowing
Only funds whose issuer publishes a full holdings file are covered, which is most large US ETFs and no mutual funds. Individual stocks you hold directly are not analysed here yet. And overlap is about what you own, not about whether it is a good idea to own it.