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With Google's Material Design style, most user interaction (tapping, pressing, etc.) creates a ripple from that point. The ripple is an expanding disc that's clipped by the widget (ignore the shadow):
Google's demos never mention what should happen when you tap a button with an existing ripple; their Polymer library supports multiple ripples, but it doesn't strictly follow all Material Design guidelines either.
Should there be multiple ripples per widget?
Since Google's Material design is about (among other things) a kind of simulated physics to provide feedback and consistency, my guess is that a ripple for every touch is best.
Often they (Google, Apple, etc.) don't think of everything in the first spec, or the first guideline docs, and there are questions such as yours that need to resolved in later specs, and my guess is the resolution will recommend multiple ripples because it's consistent with the fake physics and provides useful feedback.
