fix: reject duplicate initialize on existing sessions #2912

Job	Run time
checks / pre-commit	44s
checks / readme-snippets	15s
checks / test (3.10, locked, ubuntu-latest)	1m 3s
checks / test (3.12, lowest-direct, windows-latest)	1m 36s
checks / test (3.11, locked, windows-latest)	1m 37s
checks / test (3.11, lowest-direct, ubuntu-latest)	1m 3s
checks / test (3.10, locked, windows-latest)	1m 34s
checks / test (3.12, lowest-direct, ubuntu-latest)	1m 8s
checks / test (3.13, lowest-direct, windows-latest)	1m 29s
checks / test (3.11, locked, ubuntu-latest)	59s
checks / test (3.12, locked, windows-latest)	1m 29s
checks / test (3.13, locked, ubuntu-latest)	52s
checks / test (3.11, lowest-direct, windows-latest)	1m 39s
checks / test (3.13, lowest-direct, ubuntu-latest)	1m 0s
checks / test (3.14, locked, windows-latest)	2m 58s
checks / test (3.12, locked, ubuntu-latest)	1m 12s
checks / test (3.13, locked, windows-latest)	1m 28s
checks / test (3.14, lowest-direct, windows-latest)	1m 20s
checks / test (3.10, lowest-direct, ubuntu-latest)	1m 4s
checks / test (3.10, lowest-direct, windows-latest)	1m 45s
checks / test (3.14, locked, ubuntu-latest)	1m 46s
checks / test (3.14, lowest-direct, ubuntu-latest)	1m 1s
all-green	5s
	29m 7s

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