How do you look up the last entered text value in a column to compare against the next one

Hi there,

here is my following question/problem. In column F I have various data. It either contains "w", "x" or is blank. Want I am looking for is a function in column G that will look up the adjecent cell in column F and if it isn't blank then compare it to the last entered value in column F to see if they are the same or not. If they are the same, I need to to notify me of this, something like changing the "x" to "xx" to signify that the last value was also an "x". Then if the next value is also an "x" it would ready "xxx" an so forth. If they don't match, it can simply return the value in the adjecent cell.

Appriciate the time and effort.

Cheers,
Matt