Referencing function value inside CASE..WHEN

Hi,

In the following statement, does the custom function get executed twice?

SELECT
MY_FUNCTION(...),
CASE WHEN MY_FUNCTION(...) = '...' THEN '...' ELSE '...' END
...
...

If so, is there a way I can make it execute once? I tried this but it gave me a "Column my_function does not exist" error.

SELECT

MY_FUNCTION(...) AS my_function,

CASE WHEN my_function = '...' THEN '...' ELSE '...' END

...

...

Thank you in advance.