Bash: how to use sed to replace only the last occurence in a file?

Question

Having a file containing repeated commented lines like:

# ScriptAlias /cgi-bin/ "somepath"
# ScriptAlias /cgi-bin/ "otherpath"

I want to add a line only after the last occurence resulting in

# ScriptAlias /cgi-bin/ "somepath"
# ScriptAlias /cgi-bin/ "otherpath"
ScriptAlias /cgi-bin/ "mypath"

To do so I'm using this command:

sed -i 's:^\(.*ScriptAlias /cgi-bin/.*\):\1 \nScriptAlias /cgi-bin/ "mypath":' file

But this results in adding my line after each occurence like:

# ScriptAlias /cgi-bin/ "somepath"
ScriptAlias /cgi-bin/ "mypath"
# ScriptAlias /cgi-bin/ "otherpath"
ScriptAlias /cgi-bin/ "mypath"

How can I tell sed to replace only the last occurence?

EDITED:
If there's no way to solve it using sed (as said in comments), please provide alternatives reaching the same result, thanks.

EDITED:
The repeated lines can be separeted and with other lines between them like

# ScriptAlias /cgi-bin/ "somepath"
# ScriptAlias /cgi-bin/ "otherpath"

# ScriptAlias /cgi-bin/ "another-path"
ScriptAlias /foo/ "just-jump"
# ScriptAlias /cgi-bin/ "that's the last"

Answer 1

Use tac so you print your new line the first time you see the pattern:

tac file | awk '/ScriptAlias/ && ! seen {print "new line"; seen=1} {print}' | tac