So is the problem that you can't copy and paste the event key in the same animation?
Or that copy and paste aren't working properly in the string textbox?
Did you try these steps (assuming your sample names)? :
(1) Make a "sound_foo" event.
(2) Select the animation named double foo.
(3) Key the sound_foo event anywhere in the timeline of double foo with the correct string.
(4) Make sure the key you made is selected.
(5) CTRL/CMD + C
(6) Move the timeline cursor to another time.
(7) CTRL/CMD + V
I mean it won't solve the textbox issue if it is a textbox issue, but I think it should achieve what you wanted (assuming you wanted two or more event keys of the same event at different times that have the same string value)
I can confirm there's a textbox issue (OS X Mavericks).
Pasting text in the string textbox while an event key is selected makes the text appear but:
(1) Pressing enter
does nothing
(2) The red dot next to the event in the tree doesn't turn orange.
(3) Clicking anywhere to change focus away from the textbox makes the text disappear.
One workaround:
Press the left or right key after pasting.
This causes the red dot to turn orange, allowing you to key the change properly. This also causes the text to persist if you focus away.
I guess the enter key really does nothing though.