I keep seeing the same portfolio bug: VOO for large caps, VTI for total market, QQQ for growth. Feels diversified. You own Apple and Microsoft three times and call it a strategy. An ETF overlap analyzer app promises to catch this from your phone, and the good news is the math is simple enough that the app wrapper barely matters.
Overlap is just shared holdings measured by weight. Two funds each holding 7% Apple contribute real overlap. Two funds each holding 0.02% of some micro-cap don't. Any ETF holdings overlap checker tool worth using shows overlap by weight, uses current holdings instead of a stale snapshot, and lets you enter your whole portfolio rather than comparing two tickers in a vacuum. An ETF overlap analyzer app, iOS and Android both, that only compares pairs is a demo, not a tool.
The phone part is genuinely convenient for exactly one scenario: you're about to buy a fund and want a ten-second sanity check against what you hold. For the real audit, the kind where you discover three funds are one fund, use a full screen and real data. An ETF overlap checker on the web, or a spreadsheet if you're that person. And overlap doesn't respect borders. Check the international funds against the US funds. The multinationals recur.
Be skeptical of analyzers that are really marketing funnels. If the free check requires your email and ends in a newsletter pitch, the product is you. The math wants to be free, and mostly is.
Run the check before every buy, not after. ETFCompare computes ETF holdings overlap from actual holdings data for 108 ETFs, so you see the redundancy before you pay for it. Start at etfcompare.fyi.