Luckily I've never encountered this, in fact I've had the opposite where my G&Ts get stronger with each pour.
My last flight in Y, (Yvr-Yyz) I had the FAs search the plane looking for enough glenlivet so I could have multiple doubles.
Only time I've ever had a problem was with WS when the FA told me it was a Transport Canada rule that they weren't allowed to serve doubles.