Drop Down Lists that use the same list but removes the previous selection

I am trying to create a form that has 8 drop down lists. They are all pulling from the same list, but I  want when you open the second drop down list, you not be able to select the same as before. Simple example is as follows;

DDL1 : 1, 2, 3, 4 . User choses 3.

DDL2 : 1, 2, 4. User choses 1.

DDL3: 2, 4. User choses 2.

I have included the basic form I am working with so the complication of this can be seen. But I do not even know where to start as far as coding goes.