In many modern double-coil ignition systems (or older, like the Harley-Davidsons) the spark plug does indeed fire twice. However, this isn't usually harmful in any way. The only problem I can think of is a slightly quicker wear of the plug electrodes, but modern some modern spark plugs can be quite long-living ones.
The old engines (with a conventional points-and-distributor system) or some modern ones with direct ignition (with a separate coil for each spark plug plus the brain box) fire the spark only at determined position in compression stroke.
Hobbie