I suspect that it only shrinks once. The reason I say this is that I can, but don’t often, make clothes. One of the first things one learns is to always pre-shrink your material before cutting out the pattern. This entails running it through the wash cycle it will normally be washed on, no soap required, and running it through the dryer cycle you will normally use. Otherwise, your final product will shrink, if it’s the type of material that will, the first time you wash it.
I guess it’s too late for this shirt, but in the future I would suggest reading the label and looking up the individual fabrics content to determine if it will shrink. Synthetics won’t, natural fibers, like cotton and wool, will.