Why does SPY have two different symbols and different prices for the same strike price in the same month?
A lot of stocks have have different options symbols. They call them Standard and Non standard. They use them when companies have stock splits or pay out dividends that were not previously announced.
To expand on what rluser said, certain underlying securities (see CBOE site for complete list) have quarterly options that expire on the last business day of each calendar quarter. For SPY, you'll see the ROQ.. prefix in Mar, Jun, Sep, and Dec. expiration months. There are even weeklys on a few indices (but not SPY), but unfortunately they're not as heavily traded as SPY options.