If using reusing stock ID11 and making the 10% extra time allowance per film then you MUST return the used dev to the stock. The remaining stock effectively acts as a replenisher for the used portion. However I much prefer using the one shot dilution method as it gives consistent results. If you are doing the dilution method you need to make sure you either use a collapsible storage bottle or use an inert gas layer on top of the remaining stock to prevent oxidation.
Quote from data sheet
If a series of individual films is being developed in
a spiral tank using 1 litre of stock ID-11 or
MICROPHEN or PERCEPTOL, compensate for the
loss of developer activity after developing the first
film by increasing the development time 10% for
each successive film, (see table below). This
method of time adjustment relies on the used
developer (250 -300ml for one film), being
poured back into the stock bottle and mixed with
the fresh unused part of the developer before
processing the next film. When using spiral tanks
this helps to give more consistent results by
reducing the risks of problems due to solution
losses and the restraining effect of the by-products.