Is the "what" necessary in some sentences with the "as as " structure?

This usage of what is:

what ... 2b: that which : the one or ones that

• no income but what he gets from his writings

[Merriam-Webster]

So

• The strawberry milkshake I ordered has twice as much strawberry flavouring as what I normally receive.

=

• The strawberry milkshake I ordered has twice as much strawberry flavouring as that which I normally receive [has/does].

Even the version with 'what' sounds slightly unnatural; I'd say omitting 'what' leads to loss of idiomaticity. It certainly adds ambiguity, with the 'normally receive per day' etc reading probably the default.