Fix handling of duplicate dependencies

poetry/puzzle/provider.py

@@ -399,6 +399,28 @@ class Provider:
                     dependencies.append(list(by_constraint.values())[0][0])
                     dependencies.append(list(by_constraint.values())[0][0])
                     continue
                     continue
 
 
+                # We leave dependencies as-is if they have the same
+                # python/platform constraints.
+                # That way the resolver will pickup the conflict
+                # and display a proper error.
+                _deps = [value[0] for value in by_constraint.values()]
+                seen = set()
+                for _dep in _deps:
+                    pep_508_dep = _dep.to_pep_508()
+                    if ";" not in pep_508_dep:
+                        _requirements = ""
+                    else:
+                        _requirements = pep_508_dep.split(";")[1].strip()
+
+                    if _requirements not in seen:
+                        seen.add(_requirements)
+
+                if len(_deps) != len(seen):
+                    for _dep in _deps:
+                        dependencies.append(_dep)
+
+                    continue
+
                 # At this point, we raise an exception that will
                 # At this point, we raise an exception that will
                 # tell the solver to enter compatibility mode
                 # tell the solver to enter compatibility mode
                 # which means it will resolve for each minor
                 # which means it will resolve for each minor

tests/puzzle/test_solver.py

@@ -819,6 +819,33 @@ def test_solver_duplicate_dependencies_different_constraints(solver, repo, packa
     assert op.package.requirements == {"python": ">=3.4"}
     assert op.package.requirements == {"python": ">=3.4"}
 
 
 
 
+def test_solver_duplicate_dependencies_different_constraints_same_requirements(
+    solver, repo, package
+):
+    package.add_dependency("A")
+
+    package_a = get_package("A", "1.0")
+    package_a.add_dependency("B", {"version": "^1.0"})
+    package_a.add_dependency("B", {"version": "^2.0"})
+
+    package_b10 = get_package("B", "1.0")
+    package_b20 = get_package("B", "2.0")
+
+    repo.add_package(package_a)
+    repo.add_package(package_b10)
+    repo.add_package(package_b20)
+
+    with pytest.raises(SolverProblemError) as e:
+        solver.solve()
+
+    expected = """\
+Because a (1.0) depends on both B (^1.0) and B (^2.0), a is forbidden.
+So, because no versions of a match <1.0 || >1.0
+ and root depends on A (*), version solving failed."""
+
+    assert str(e.value) == expected
+
+
 def test_solver_duplicate_dependencies_sub_dependencies(solver, repo, package):
 def test_solver_duplicate_dependencies_sub_dependencies(solver, repo, package):
     package.add_dependency("A")
     package.add_dependency("A")