Add check to avoid overriding existing keys #1061

Job	Run time
build (stellar-cli, stellar, ubuntu-20.04, x86_64-unknown-linux-gnu)	14m 32s
build (stellar-cli, stellar, ubuntu-20.04, aarch64-unknown-linux-gnu)	16m 22s
build (stellar-cli, stellar, macos-14, x86_64-apple-darwin)	9m 9s
build (stellar-cli, stellar, macos-12, aarch64-apple-darwin)	24m 36s
build (stellar-cli, stellar, windows-latest, x86_64-pc-windows-msvc, .exe)	20m 17s
build (soroban-cli, soroban, ubuntu-20.04, x86_64-unknown-linux-gnu)	14m 54s
build (soroban-cli, soroban, ubuntu-20.04, aarch64-unknown-linux-gnu)	16m 9s
build (soroban-cli, soroban, macos-14, x86_64-apple-darwin)	12m 27s
build (soroban-cli, soroban, macos-12, aarch64-apple-darwin)	24m 1s
build (soroban-cli, soroban, windows-latest, x86_64-pc-windows-msvc, .exe)	21m 17s
	2h 53m 44s

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