Remove Insert Option item from the available list if selected item already has children of the specific item

Sitecore Insert Option

Rules -> Insert Options -> Rules and create a new rule for "item xyz"
Then follow along with the code below to set up the rule

'where' the item is the 'home' item or one of its subitems
'and' 'except where' the item template is 'item xyz'
'and' 'except where' the result of query './*[(@@templateid='{item-xyz-template-ID}')]' exists
add 'item xyz' insert option

Basically: No parent should have more than one item with template id of "item xyz" as a child, 
so if it already does, remove "item xyz" as an option in Insert Options list