Fixing overwriting sub-steps in 'athenopts' for merging

The bug appeared when multiple sub-steps are used in "athenaopts" e.g: "HITtoRDO:--nprocs=$ATHENA_CORE_NUMBER" "HITtoRDO:--threads=0" "ESDtoDPD:--threads=0" (usage of the option is discussed here : ATLMCPROD-9279) At this merging stage, the code overwrites "athenaopts" sub-steps with the first sub-step from the loop and this causes that the remain sub-steps become unknown to "myargdict" during loop. Despite that, the logic is to search through all sub-steps and look for those options manipulating 'nprocs'.

The fix resolves the bug. Since there are multiple "--threads" in the athenaopts (in this case), during merging sub-steps, we should prevent having more than one of them in the new option list (because later transforms will raise an error regarding multiple "threads" options).