Richard,
By the way -- I should have said this earlier, but I forgot to --
What you did was the reason for the error message.
You had two capacitors both named "Cspark, inside of the same subcircuit.? LTspice needs to alter that name when it "expands" the subcircuit into the expanded SPICE netlist.? It does that by inserting extra characters into the name.? In your case, it turned "Cspark" into "C:U2:spark".? So what it did was insert? "U2" in the middle of the name, after the first character which must be a "C" to indicate that it is a capacitor.? In your case, it found that Cspark was inside of a subcircuit which had the instance name "U2", so "U2" was the text that it inserted between the "C" and the "spark".? The extra colons are how it separated U2 from the rest of the name.
I am just explaining how "Cspark" got turned into "C:U2:spark".? "CLU2:spark" "C:U2:spark" is LTspice's way of saying "the capacitor you named 'Cspark' which I found inside of the subcircuit instance named 'U2'."
(And I need to proof-read more carefully what I type.? I made careless errors which I didn't catch before clicking the "Send" button.)
Andy
