fix duplicated elements in vasp/xml

[njzjz]

Fix #480.

250 doesn't handle this situation.

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Comparison is base (e52b699) 82.87% compared to head (be99556) 82.87%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## devel #575 +/- ## ======================================= Coverage 82.87% 82.87% ======================================= Files 74 74 Lines 6606 6606 ======================================= Hits 5475 5475 Misses 1131 1131 ``` | [Files](https://app.codecov.io/gh/deepmodeling/dpdata/pull/575?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=deepmodeling) | Coverage Δ | | |---|---|---| | [dpdata/vasp/xml.py](https://app.codecov.io/gh/deepmodeling/dpdata/pull/575?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=deepmodeling#diff-ZHBkYXRhL3Zhc3AveG1sLnB5) | `70.78% <100.00%> (ø)` | |

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