@Override
			void check(PcodeParser parser, String name) {
				SleighSymbol symbol = parser.findSymbol(name);
				if (symbol != null) {
					throw new SleighException(
						"Duplicate symbol '" + name + "': Already defined by the language");
				}
			}
The name cannot already be defined by the language so it is free
Ghidra/Features/SystemEmulation/src/main/java/ghidra/pcode/struct/DefaultVar.java