implemented duplicate checking, part 1
added option to skip duplicates

3 MINUS 2 is now duplicate of 2 REVERSE_MINUS 3
3 DIVIDE 2 is now duplicate of 2 REVERSE_DIVIDE 3
test duplicate check part 2
modified gitignore
duplicate detection improvement, now detects 4x2 == 2x4 and 4+2 == 2+4