What are the current overlaps of the same stocks held within different ETFs?

Generally, the only stocks which will have overlap are value stocks and small-cap stocks.

We've employed evidence-backed optimizations that tilt a recommended portfolio toward value stocks and investments with a smaller market capitalization. To achieve this tilt, we use a whole U.S. stock-market ETF for our core holding, and then invest in three additional value-based ETFs for large-cap, mid-cap, and small-cap value equity exposure, respectively.

Stocks which fall into these three ETFs will have double exposures, which is exactly what we want–to be overweight in them compared to the broad market.

See the full list of ETFs in the Betterment Portfolio.