Alpha String Formula - Remove Duplicates When Starting In Cell Going Downward

I have a string of letters in cell column L20 through L3019.

The letters are A, B, C, and R . No other letters.

I wanted to find out if it is possible to remove any duplicates when starting in cell L20 going downward.

For example:

L20 = B
L21 = A
L22 = A
L23 = B
L24 = C
L25 = C
L26 = C
L27 = C
L28 = B
L29 = R
L30 = A
L31 = C
L32 = C
L33 = C
L34 = R
L35 = R
L36 = B
L37 = B

In the range L20 through L37 all the letters have appeared so my goal is to create a formula where the outcome is BACR. Starting with the first letter B in cell L20

Cell range L20 through L37. Looking for the next letter which would be either A,C, or R. Since we already used the letter B. In this case it's the letter A
ABCR

Cell range L20 through L37. Looking for the next letter which is the C or R. In this case it's the letter C
CBRA

Finally range L20 through L37. Looking for the letter R which is in cell L29.
RACB