How to deal with reused normal sample?

[ligot21]

Thank you for your work. It helps a lot.
Sometimes more than one tumor samples will refer to a same control. For example, we might have a pretreatment tumor sample (tumor_pre), a post treatment tumor sample (tumor_post), a post treatment lymph metastasis (tumor_ln_post) referring to a same blood control (control_sample). Now I put the working directory and the 'tumor_control_samples.txt' file in this way. It seems fine and works well:

$ ls
configuration.py  control_sample  tumor_control_samples.txt  tumor_ln_post  tumor_post  tumor_pre

$ cat tumor_control_samples.txt 
tumor_pre control_sample
tumor_post control_sample
tumor_ln_post control_sample

However, I am not sure whether this will cause any problems such as unnecessary recalculation or bias in the output file. For example, I know panel of normals (PONs) will be calculated by GATK, will one normal sample be used multiple times for calculation of PONs and cause problems?