diff --git a/.pre-commit-config.yaml b/.pre-commit-config.yaml
index f70b6e7..a10e3d9 100644
--- a/.pre-commit-config.yaml
+++ b/.pre-commit-config.yaml
@@ -13,7 +13,7 @@ repos:
       - id: end-of-file-fixer
       - id: trailing-whitespace
   - repo: https://github.com/codespell-project/codespell
-    rev: v2.1.0
+    rev: v2.3.0
     hooks:
       - id: codespell
   - repo: https://github.com/PyCQA/pydocstyle
@@ -22,7 +22,7 @@ repos:
       - id: pydocstyle
         exclude: ^(functional_tests|tests|doc|examples|setup.py)
   - repo: https://github.com/PyCQA/flake8
-    rev: 4.0.1
+    rev: 5.0.4
     hooks:
       - id: flake8
         exclude: ^(functional_tests|tests|doc)
diff --git a/morph_tool/morphdb.py b/morph_tool/morphdb.py
index 42d4d3f..963d44b 100644
--- a/morph_tool/morphdb.py
+++ b/morph_tool/morphdb.py
@@ -264,7 +264,7 @@ def from_folder(cls,
         Args:
             morphology_folder: a folder containing morphologies
             mtypes: a sequence of 2-tuples (morphology name, mtype)
-            label: (optional) a group label to be used to identify the morphlogies from this folder
+            label: (optional) a group label to be used to identify the morphologies from this folder
             extension: Specify the morphology format to consider, if the folder contains multiple
                 formats
 
@@ -373,7 +373,10 @@ def __add__(self, other):
     def __iadd__(self, other):
         """Overloaded method."""
         if isinstance(other, MorphDB):
-            self.df = pd.concat([self.df, other.df]).drop_duplicates()
+            self.df = pd.concat(
+                [self.df, other.df],
+                ignore_index=True,
+            ).drop_duplicates(["name", "mtype", "layer"])
             MorphDB._sanitize_df_types(self.df)
         else:
             raise TypeError(f'Must be MorphDB or a sequence of MorphInfo, not {type(other)}')