drop the old data type with the same name and throw warning

dpdata/system.py

@@ -1,6 +1,7 @@
 # %%
 import glob
 import os
+import warning
 from copy import deepcopy
 from typing import Any, Dict, Optional, Tuple, Union
 
@@ -963,7 +964,13 @@ def register_data_type(cls, *data_type: Tuple[DataType]):
         *data_type : tuple[DataType]
             data type to be regiestered
         """
-        cls.DTYPES = cls.DTYPES + tuple(data_type)
+        all_dtypes = cls.DTYPES + tuple(data_type)
+        dtypes_dict = {}
+        for dt in all_dtypes:
+            if dt.name in dtypes_dict:
+                warnings.warn(f"Data type {dt.name} is registered twice; only the newly registered one will be used.", UserWarning)
+            dtypes_dict[dt.name] = dt
+        cls.DTYPES = tuple(dtypes_dict.values())
 
 
 def get_cell_perturb_matrix(cell_pert_fraction):