[BUGFIX] Don't run processIncludes more than once

processIncludes() is run "mandatory" from the stack ->start()
->runThroughTemplates() and doing all the work that needs to be done.

Resolves: #54903
Releases: 6.2
Change-Id: Id2be1a25aa23a05849297315c74358a9ef4ff9bb
Reviewed-on: https://review.typo3.org/26748
Reviewed-by: Alexander Opitz
Tested-by: Alexander Opitz
Reviewed-by: Oliver Klee
Reviewed-by: Markus Klein
Tested-by: Markus Klein