PowerBI Service Account granting duplicate execute permissions on stored procedure almost constantly
I have worked out what is happening here.  tl/dr - There was a missing batch separator between a stored procedure and the associated GRANT statement.
If I run:
SELECT * 
FROM sys.sql_modules 
WHERE definition LIKE '%GRANT EXECUTE ON \[dbo\].\[ExtendEditSessionLifetime\] TO RSExecRole%' ESCAPE '\'

I can see stored procedures that have that grant statement within them.
This returned one record and the definition field begins
CREATE PROC [dbo].[ExtendEditSessionLifetime]      @EditSessionID varchar(32)...... 

If I navigate to this stored procedure in the object tree and right click and script it out, I can see text of the stored procedure:
CREATE PROC [dbo].[ExtendEditSessionLifetime]
    -- params
AS
BEGIN
    --code
END
    
GRANT EXECUTE ON [dbo].[ExtendEditSessionLifetime] TO RSExecRole
GO

I wasn't aware of this but it seems that when you create a stored procedure, you can place a GRANT after the END and it becomes part of the definition (I thought it had to be part of the BEGIN .... END construct (when there is one)
Therefore, every time ExtendEditSessionLifetime is run, it is re-granting permissions to RSExecRole.