fix: deepcopy should copy same table exactly only once

[Revolyssup]

Description

Fixes # (issue)
The current deepcopy, when it finds that the original table has already been copied, will directly use the original table. This causes the copied table and the original table to share one table, which does not conform to the semantics of deepcopy. After fixing, the same table will be copied only once.

Checklist

• I have explained the need for this PR and the problem it solves
• I have explained the changes or the new features added to this PR
• I have added tests corresponding to this change
• I have updated the documentation to reflect this change
• I have verified that this change is backward compatible (If not, please discuss on the APISIX mailing list first)