I have made the following custom list.

GR2-E7
GR4-E7
REST
G-E7
GR2-E7
G-E7
G-E7
REST
G-E7
GR2-E7
GR4-E7
REST
G-E7

When using the fill method by dragging from the first cell (i.e.
GR2-E7) excel reaches the second GR2-E7 it should goto G-E7, G-E7, etc.

However, it then follows with GR4-E7.

Note: I am dragging from left to right.

Any ideas as I have loads of rows to fill, and I dont want the users to
do this manually.

Thanks in advance.