Add check to avoid overriding existing keys #807

Job	Run time
build (stellar-cli, stellar, ubuntu-20.04, x86_64-unknown-linux-gnu)	14m 49s
build (stellar-cli, stellar, ubuntu-20.04, aarch64-unknown-linux-gnu)	16m 2s
build (stellar-cli, stellar, macos-14, x86_64-apple-darwin)	10m 42s
build (stellar-cli, stellar, macos-12, aarch64-apple-darwin)	20m 14s
build (stellar-cli, stellar, windows-latest, x86_64-pc-windows-msvc, .exe)	19m 45s
build (soroban-cli, soroban, ubuntu-20.04, x86_64-unknown-linux-gnu)	15m 18s
build (soroban-cli, soroban, ubuntu-20.04, aarch64-unknown-linux-gnu)	16m 19s
build (soroban-cli, soroban, macos-14, x86_64-apple-darwin)	10m 50s
build (soroban-cli, soroban, macos-12, aarch64-apple-darwin)	17m 20s
build (soroban-cli, soroban, windows-latest, x86_64-pc-windows-msvc, .exe)	21m 7s
	2h 42m 26s

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