It's been all systems go for the S&P 500 index this year. Indeed, with a year-to-date return of 17%, a rebound from last year's dreadful 18% decline is almost complete.

However, there is one underlying issue with the index that has me concerned. In fact, I think it's led some investors to become overly exposed to certain companies but more on that later.

Let's start by examining what's going on within the S&P 500, and why it matters to investors.

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Source Fool.com