awk causing memory leak?

I am using awk to process a huge data file (about 100 million lines).

This is my expression-

tac <myfile> | awk -F',' 'OFS=\",\" {if(/^#/){print;}else{\$1=tolower(\$1);if(!seen[\$1]++)print;}}'|")

However, I'm seeing memory leak with awk. Can someone point out the memory hog here?

Thanks!