When evaluating ETFs for your portfolio, one of things you should check is if the fund is holding stocks that you already have exposure to through other ETFs.
The pros call holding the same security across multiple funds overlap. The idea is to avoid going through the time and expense of evaluating and adding another fund if it is going to be redundant.
One way to check for overlap is to use a tool on Yahoo! Finance called Components. For example, pull up the Yahoo! finance page for Wells Fargo (WFC). Go to the navigation bar on the left side of the page and click on Company>Components. In the center of the page, Yahoo lists all of the ETFs that hold Wells Fargo as a top 10 holding.
Notice that KBE, IYG and FDL all hold over 4% of their assets in Wells Fargo. So if you already own one of these ETFs, you should either add to that position or find another ETF not on this list for diversification and to avoid the redundancy that comes with overlap.