I am not sure what you mean by the 'cup' the shop replaced but in general if the left crank arm keeps loosening and coming off you need a new crank arm.
Is the crank arm loose on what would be the spindle? or is the bottombrakcet loose in the frame? a pic or two may help
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“One morning you wake up, the girl is gone, the bikes are gone, all that's left behind is a pair of old tires and a tube of tubular glue, all squeezed out"
Sugar "Kane" Kowalczyk