Can I refreeze defrosted crumb topping?
Generally, freezing and re-freezing is not a big issue. Some things just shouldn't be frozen (certain fruits and veg), because when frozen the water contained in the cell structure expands and in turn bursts the containing structure, which causes mushiness. Meats can be refrozen with a slight moisture loss. From what I was taught by my old pastry chef, this whole notion of freezing and refreezing being dangerous was started when electric freezers were just coming into the market (first half of the 20th century), and the cold storage and ice companies were scared of the potential loss of their business (which absolutely happened). It was them that advertised that this refreezing could be detrimental to health.  Truth is, refreezing just sends the item into a (mostly) inert state, where bacteria can't harm the item by propagating.
However, your question is about the taste and how refreezing pertains to cookie dough. It's not a matter of the refreezing that has an effect on the taste, rather how long things stay in your freezer. You can thaw, refreeze, thaw, refreeze as much as you want. Happens all the time in commercial kitchens and bakeries. The key is to not make so much that it stays in the freezer too long.

Here's and article I found, haven't looked for any others:

Safe food practices