Question: [![enter image description here](https://i.stack.imgur.com/eaUmw.png)](https://i.stack.imgur.com/eaUmw.png)

As you can see in the image above I have an example of characters which can be entered. However in that row 'A' can only be entered once. How can I implement this rule?

Answer: 
Use data validation on the row with this formula:

```
=COUNTIF(1:1,A1)=1

```

change `1:1` as required for the row you need.

[![enter image description here](https://i.stack.imgur.com/SPSAI.png)](https://i.stack.imgur.com/SPSAI.png)
You can do this using Data Validation.

* Select the complete row say Row 1
* In the `Data` Menu, click `Data Validation` in `Data Tools` Group
* Under `Settings` tab, select `Custom` in `Allow` drop down list
* In the `Formula` box, enter `=COUNTIF(1:1,A1)=1` and click `OK` (in the formula 1 represents the `Row 1`)

See the image below:

[![enter image description here](https://i.stack.imgur.com/Ou0Rt.png)](https://i.stack.imgur.com/Ou0Rt.png)

If you'll try to enter already existing character then the following message will pop up.

[![enter image description here](https://i.stack.imgur.com/GGGav.png)](https://i.stack.imgur.com/GGGav.png)

If you specifically want only letter "A" not to repeat then change the formula to `=COUNTIF(1:1,"A")=1`