Difference between ! vs !! vs * in /etc/shadow

The second field in the Linux /etc/shadow file represents a password. However, what we have seen is that: 1. Some of the password fields may have a single exclamation :!:..... 2. Some of the password fields may have a double exclamation :!!:..... 3. Some of the password fields may have an asterisk sign :*:..... By some research on internet and through this thread , I can understand that * means password never established, ! means locked. Can someone explain what does double exclamation (!!) mean? and how is it different from (!)?