Conditional Formatting using multiple conditions to find duplicates

I am having trouble getting a formula to highlight duplicate values. The duplicate value is considered a duplicate when two values in two separate columns are the same for both columns. In other words, values in column A and column B can be the same, but they cannot be the same for both column A and column B. I tried to use =AND($A1=$A1:$A340,$B1=$B1:$B340), but this is only highlighting the blank rows below my desired range. Any ideas?