This is the 6294th most frequent German word.
doppel
Double.
Here, 'Doppel' modifies 'Portion', meaning 'double' or 'twice the amount'.
Er kaufte eine Doppelportion Eis.
He bought a double portion of ice cream.
In this context, 'Doppel' refers to an object designed for two, as in 'Doppelbett', meaning 'double bed'.
Das Doppelbett ist sehr bequem.
The double bed is very comfortable.
Here, 'Doppel' is part of the term 'Doppelgänger', referring to a person who looks very similar to another.