Allow values in a cell only if such values do not exist in the same column or row

Hi! I have a table which I wish to fill in with the values given in another table. The condition is that if for example, I enter C on cell G4, Excel, somehow should prevent or warn me that C is not allowed on G4 because that value is already in F4 (same row), it should also check the column. e.g. If I try to enter M on cell G4, Excel should inform me that M alredy exists on that column (G8).

I'm a bit clueless on how to start. I thought about VBA, but I would prefer to use a formula.

Any help will be greatly appreciated.

Thank you very much Jazzer! But how can I restrict the values entered to the list on column L only?

I added the OR:
=AND(COUNTIF(E$2:E$8,E2)<=1,COUNTIF($E2:$H2,E2)<=1,OR(E2=L2,E2=L3,E2=L4,E2=L5,E2=L6,E2=L7,E2=L8,E2=L9,E2=L10))

Could you show me a better way to do it?

That will not work for your other cells. For that to work your references to L have to be absolute:

....E2=$L$2, E2=$L$3....

Here's another way:

=AND(COUNTIF(E$2:E$8,E2)<=1,COUNTIF($E2:$H2,E2)<=1,COUNTIF($L$2$L$10,E2)=1)

Make sure you include all the $ signs as shown

I get it. Thank so much! It seems to be working perfectly well