You must be a pact of the chain warlock to be able to re-summon Shovel right?
Shovel X Generic Quasit[edit source]
I sold the scroll way back in Act 1, without using it. WHen I found out about Shovel I tried adding the scroll with Cheat Engine, but it only summons a generic Quasit, like the one from the warlock pact. Summoning Shovel might be tied to an specific location or trigger in the first act.