Description:
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PS(define_sid) = 1;
PS(send_cookie) = 1;

in session.c is before

if (PS(session_status) != php_session_none)

thus SID is created and cookie is sent uncoditionally in case of second call of session_start().

Reproduce code:
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session_start();
session_start();
echo SID;

Expected result:
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Nothing in case of sent cookie.

Actual result:
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PHPSESSID=...