Set `B2`

to

```
=IF(A2=A3, 1, -2)
```

and set `B3`

to

```
=IF($A2=$A3, B2, IF($A3=$A4, IF(B2>0,3-B2,B2+3), IF(B2>0,-B2,B2)))
```

and drag it down to `B30`

(or the last row that contains data, whatever that is).
This will evaluate to a positive number
if this row is part of a duplicate value group
and a negative value if it isn’t
(i.e., if Column `A`

contains a unique value).
Throughout the first duplicate value group, Column `B`

will be 1;
throughout the second, it will be 2;
throughout the third, it will be 1 again, and so on (alternating).
On rows with unique values, Column `B`

will contain
the negative of the value of the most recent duplicate value group.

### Step by step:

First row:

- If
`A2=A3`

, then Rows `2`

and `3`

are part of the same duplicate value group,
and so `B`

should be 1, because we want the first group to be numbered 1.
Otherwise (if `A2`

≠`A3`

), Row `2`

is *not* part of a duplicate value group
(we don’t know yet about Row `3`

), so it should have a negative value.
We make it -2, so that the first duplicate value group (when we find it)
will be numbered 1.

Subsequent rows:

- If
`A2=A3`

,
then this row and the previous one are part of the same duplicate value group,
and so `B`

should be the same as it was on the previous row.
- Otherwise (if
`A2`

≠`A3`

), if `A3=A4`

, then this row and the next one
are the first two rows of a new duplicate value group,
and so `B`

is `IF(B2>0,3-B2,B2+3)`

:
- If
`B2>0`

,
then the previous row was the last row of a different duplicate value group.
So we want to alternate values between 1 and 2 —
if the previous row was 1, we want this one to be 2, and vice versa.
The expression `3-B2`

implements this alternating behavior:
`3-1`

is 2 and `3-2`

is 1.
- Otherwise (if
`B2`

≤`0`

), the previous row has a unique value in Column `A`

,
and Column `B`

has the negative of the `B`

value of the most recent group.
Again, we want to alternate values between 1 and 2 —
if the previous row was -1, we want this one to be 2, and vice versa.
We get this with `B2+3`

: `-1+3`

is 2 and `-2+3`

is 1.

Otherwise (if `A3`

≠`A4`

),
then this row is a unique value row, and so `B`

is `IF(B2>0,-B2,B2)`

:

- If
`B2>0`

, then the previous row
was the last row of a different duplicate value group,
and we want this row’s `B`

value to be the negative of that.
- Otherwise (if
`B2`

≤`0`

), the previous row is also a unique value row,
so we want to keep the same `B`

value.

I guess I could have said `-ABS(B2)`

here.

So now, obviously, you use conditional formatting
to color cells red if the value in Column `B`

is 1 and green is it is 2.