diff --git a/grass-gis-addons/m.analyse.trees/analyse_trees_lib/analyse_trees_lib.py b/grass-gis-addons/m.analyse.trees/analyse_trees_lib/analyse_trees_lib.py
index 8835f3b..73a58d6 100644
--- a/grass-gis-addons/m.analyse.trees/analyse_trees_lib/analyse_trees_lib.py
+++ b/grass-gis-addons/m.analyse.trees/analyse_trees_lib/analyse_trees_lib.py
@@ -448,3 +448,26 @@ def compute_ndvi_neighbors(ndvi, nprocs, memory, rm_rasters):
     rm_rasters.append(f"{ndvi_split}_max1")
     rm_rasters.append(f"{ndvi_split}_max2")
     return f"{ndvi_split}_max2"
+
+
+def calculate_nd(band1, band2, output):
+    """Calculate NDWI or NDGB if does not exists
+    Args:
+        band1(string): Name of green raster map
+        band2(string): Name of nir (for NDWI) or blue (for NDGB) raster map
+        output(string): Name for output NDWI or NDGB raster map
+    """
+    if not grass.find_file(name=output, element="cell")["file"]:
+        grass.mapcalc(
+            f"{output} = round(127.5 * (1.0 + float({band1} - {band2}) / float({band1} + {band2})))"
+        )
+    else:
+        grass.warning(
+            _(
+                f"Map <{output}> already exists."
+                "If you want to recalculate all existing data use --o "
+                f"and if you only want to recalculate {output}, "
+                "please delete the map first with:\n"
+                f"<g.remove -rf type=raster name={output}>"
+            )
+        )
diff --git a/grass-gis-addons/m.analyse.trees/r.trees.mltrain/r.trees.mltrain.py b/grass-gis-addons/m.analyse.trees/r.trees.mltrain/r.trees.mltrain.py
index 2247ef4..1a6e6dc 100644
--- a/grass-gis-addons/m.analyse.trees/r.trees.mltrain/r.trees.mltrain.py
+++ b/grass-gis-addons/m.analyse.trees/r.trees.mltrain/r.trees.mltrain.py
@@ -216,6 +216,7 @@ def main():
     sys.path.append(path)
     try:
         from analyse_trees_lib import (
+            calculate_nd,
             create_nearest_pixel_ndvi,
             set_nprocs,
             test_memory,
@@ -258,15 +259,11 @@ def main():
 
     if not ndwi:
         ndwi = "ndwi"
-        grass.mapcalc(
-            f"{ndwi} = round(127.5 * (1.0 + float({green} - {nir}) / float({green} + {nir})))"
-        )
+        calculate_nd(green, nir, ndwi)
 
     if not ndgb:
         ndgb = "ndgb"
-        grass.mapcalc(
-            f"{ndgb} = round(127.5 * (1.0 + float({green} - {blue}) / float({green} + {blue})))"
-        )
+        calculate_nd(green, blue, ndgb)
 
     if options["trees_raw_v"]:
         trees_raw_v_rast = f"trees_raw_v_rast_{os.getpid()}"
diff --git a/grass-gis-addons/m.analyse.trees/r.trees.postprocess/r.trees.postprocess.py b/grass-gis-addons/m.analyse.trees/r.trees.postprocess/r.trees.postprocess.py
index 63a321c..c29ce88 100644
--- a/grass-gis-addons/m.analyse.trees/r.trees.postprocess/r.trees.postprocess.py
+++ b/grass-gis-addons/m.analyse.trees/r.trees.postprocess/r.trees.postprocess.py
@@ -212,7 +212,11 @@ def main():
         grass.fatal("Unable to find the analyse trees library directory.")
     sys.path.append(path)
     try:
-        from analyse_trees_lib import set_nprocs, test_memory
+        from analyse_trees_lib import (
+            calculate_nd,
+            set_nprocs,
+            test_memory,
+        )
     except Exception:
         grass.fatal("analyse_trees_lib missing.")
 
@@ -256,17 +260,11 @@ def main():
 
     if not ndwi:
         ndwi = "ndwi"
-        grass.mapcalc(
-            f"{ndwi} = round(127.5 * (1.0 + float({green} - {nir}) / float({green} + {nir})))",
-            overwrite=True,
-        )
+        calculate_nd(green, nir, ndwi)
 
     if not ndgb:
         ndgb = "ndgb"
-        grass.mapcalc(
-            f"{ndgb} = round(127.5 * (1.0 + float({green} - {blue}) / float({green} + {blue})))",
-            overwrite=True,
-        )
+        calculate_nd(green, blue, ndgb)
 
     # estimate trees from nearest peak IDs and various bands
 
diff --git a/grass-gis-addons/m.analyse.trees/r.trees.traindata/r.trees.traindata.py b/grass-gis-addons/m.analyse.trees/r.trees.traindata/r.trees.traindata.py
index 2b468a8..935bc55 100644
--- a/grass-gis-addons/m.analyse.trees/r.trees.traindata/r.trees.traindata.py
+++ b/grass-gis-addons/m.analyse.trees/r.trees.traindata/r.trees.traindata.py
@@ -212,6 +212,7 @@ def main():
     sys.path.append(path)
     try:
         from analyse_trees_lib import (
+            calculate_nd,
             create_nearest_pixel_ndvi,
             set_nprocs,
             test_memory,
@@ -255,15 +256,11 @@ def main():
 
     if not ndwi:
         ndwi = "ndwi"
-        grass.mapcalc(
-            f"{ndwi} = round(127.5 * (1.0 + float({green} - {nir}) / float({green} + {nir})))"
-        )
+        calculate_nd(green, nir, ndwi)
 
     if not ndgb:
         ndgb = "ndgb"
-        grass.mapcalc(
-            f"{ndgb} = round(127.5 * (1.0 + float({green} - {blue}) / float({green} + {blue})))"
-        )
+        calculate_nd(green, blue, ndgb)
 
     # estimate trees from nearest peak IDs and various bands
 
diff --git a/grass-gis-addons/m.analyse.trees/v.trees.species/v.trees.species.py b/grass-gis-addons/m.analyse.trees/v.trees.species/v.trees.species.py
index 48dacad..aa5f2b8 100644
--- a/grass-gis-addons/m.analyse.trees/v.trees.species/v.trees.species.py
+++ b/grass-gis-addons/m.analyse.trees/v.trees.species/v.trees.species.py
@@ -207,7 +207,12 @@ def main():
         grass.fatal("Unable to find the analyse trees library directory")
     sys.path.append(path)
     try:
-        from analyse_trees_lib import reset_region, set_nprocs, test_memory
+        from analyse_trees_lib import (
+            calculate_nd,
+            reset_region,
+            set_nprocs,
+            test_memory,
+        )
     except Exception:
         grass.fatal("analyse_trees_lib missing.")
 
@@ -280,10 +285,7 @@ def main():
         grass.message(_("Computing NDWI ..."))
         ndwi = f"ndwi_{tmp_name}"
         rm_rasters.append(ndwi)
-        grass.mapcalc(
-            f"{ndwi} = round(255 * (1.0 + ( float({green} - {nir})/"
-            f"({green} + {nir}) ))/2)"
-        )
+        calculate_nd(green, nir, ndwi)
 
     grass.message(_("Classifying deciduous and coniferous trees ..."))
     classification_group = f"classification_group_{tmp_name}"