diff --git a/CaseStudy.py b/CaseStudy.py
index 8f1bb5f..614a128 100644
--- a/CaseStudy.py
+++ b/CaseStudy.py
@@ -12,7 +12,7 @@
 
 import ExcelReader
 from InOutModule import Utilities
-from printer import Printer
+from InOutModule.printer import Printer
 
 printer = Printer.getInstance()
 
@@ -196,6 +196,8 @@ def __init__(self,
                     printer.error(f"Error reading for '{attr_name}': {exc}")
                     raise exc
 
+        self.check_duplicate_lines_dPower_Network()
+
         # === SEQUENTIAL DEPENDENTS ===
         if dPower_WeightsRP is not None:
             self.dPower_WeightsRP = dPower_WeightsRP
@@ -256,6 +258,45 @@ def copy(self):
         new_self = copy.deepcopy(self)
         return new_self
 
+    def check_duplicate_lines_dPower_Network(self):
+        """
+        Check dPower_Network for duplicate and parallel lines.
+        i -> j and j -> i are considered as the same line.
+        Lines with same endpoints and circuit -> error.
+        Lines with same endpoints and different circuits-> warning.
+        :return: None
+        """
+        dPower_Network = self.dPower_Network.reset_index()
+        line_keys_without_circuit = dPower_Network.apply(lambda row: frozenset((row["i"], row["j"])), axis=1)
+        dPower_Network["_line_key_without_circuit"] = line_keys_without_circuit
+        line_keys = dPower_Network.apply(lambda row: (row["scenario"], line_keys_without_circuit[row.name], row["c"]), axis=1)
+        duplicate_lines = dPower_Network[line_keys.duplicated(keep=False)]
+
+        if not duplicate_lines.empty:
+            duplicate_line = duplicate_lines.iloc[0]
+            raise ValueError(
+                f"Duplicate network line found in (at least) scenario '{duplicate_line['scenario']}' "
+                f"for line {duplicate_line['i']} <-> {duplicate_line['j']} "
+                f"with circuit '{duplicate_line['c']}'. "
+                "If the lines should be parallel, assign different 'c' for each parallel line."
+            )
+
+        lines_with_multiple_circuits = dPower_Network[dPower_Network.groupby(["scenario", "_line_key_without_circuit"])["c"].transform("nunique") > 1]
+
+        if not lines_with_multiple_circuits.empty:
+            parallel_line = lines_with_multiple_circuits.iloc[0]  # for printing the example: use first row that belongs to a parallel line group
+            parallel_group = lines_with_multiple_circuits[
+                (lines_with_multiple_circuits["scenario"] == parallel_line["scenario"])  # same scenario
+                & (lines_with_multiple_circuits["_line_key_without_circuit"] == parallel_line["_line_key_without_circuit"])
+                ]
+            circuits = parallel_group["c"].head(2).tolist()  # show only first two circuit IDs
+
+            printer.warning(
+                f"Parallel network lines found in (at least) scenario '{parallel_line['scenario']}' "
+                f"for line {parallel_line['i']} <-> {parallel_line['j']} "
+                f"with circuits {circuits}."
+            )
+
     def equal_to(self, cs: typing.Self) -> bool:
         """
         Check if this CaseStudy is equal to another CaseStudy, checking all dataframes for equality.
diff --git a/ExcelReader.py b/ExcelReader.py
index 6ae4ab9..d940c42 100644
--- a/ExcelReader.py
+++ b/ExcelReader.py
@@ -5,7 +5,7 @@
 from openpyxl import load_workbook
 from openpyxl.utils.cell import get_column_letter
 
-from printer import Printer
+from InOutModule.printer import Printer
 
 printer = Printer.getInstance()
 
diff --git a/ExcelWriter.py b/ExcelWriter.py
index f1ee83e..9dfaccb 100644
--- a/ExcelWriter.py
+++ b/ExcelWriter.py
@@ -19,7 +19,7 @@
 if TYPE_CHECKING:
     from CaseStudy import CaseStudy
 from TableDefinition import CellStyle, Alignment, Font, Color, Text, Column, NumberFormat, TableDefinition
-from printer import Printer
+from InOutModule.printer import Printer
 
 package_directory_ExcelWriter = os.path.dirname(os.path.abspath(__file__))
 
diff --git a/nrel118-reader.py b/nrel118-reader.py
index 3ef7ec4..784f31e 100644
--- a/nrel118-reader.py
+++ b/nrel118-reader.py
@@ -2,7 +2,7 @@
 
 import pandas as pd
 
-from printer import Printer
+from InOutModule.printer import Printer
 
 printer = Printer.getInstance()
 
diff --git a/printer.py b/printer.py
index 1517c9c..19baf42 100644
--- a/printer.py
+++ b/printer.py
@@ -5,6 +5,11 @@
 from rich.console import Console
 from rich.markup import escape
 
+if __name__ == "printer":
+    raise ImportError(
+        "Import Printer as 'from InOutModule.printer import Printer', not 'from printer import Printer'"
+    )
+
 
 class Printer:
     """
diff --git a/pyproject.toml b/pyproject.toml
index a72a436..25294be 100644
--- a/pyproject.toml
+++ b/pyproject.toml
@@ -8,4 +8,7 @@ version = "0.0.0-dev"
 readme = "README.md"
 
 [tool.setuptools]
-py-modules = ['CaseStudy', 'ExcelReader', 'ExcelWriter', 'printer', 'PypsaReader', 'pypsa_helper', 'SQLiteWriter', 'TableDefinition']
\ No newline at end of file
+py-modules = ['CaseStudy', 'ExcelReader', 'ExcelWriter', 'printer', 'PypsaReader', 'pypsa_helper', 'SQLiteWriter', 'TableDefinition']
+
+[tool.pytest.ini_options]
+pythonpath = [".."]
diff --git a/tests/test_ExcelReaderWriter.py b/tests/test_ExcelReaderWriter.py
index e33f829..baa578f 100644
--- a/tests/test_ExcelReaderWriter.py
+++ b/tests/test_ExcelReaderWriter.py
@@ -2,7 +2,7 @@
 
 import ExcelReader as ExcelReader
 from ExcelWriter import ExcelWriter
-from printer import Printer
+from InOutModule.printer import Printer
 
 printer = Printer.getInstance()