diff --git a/src/pybuild_deps/compile_build_dependencies.py b/src/pybuild_deps/compile_build_dependencies.py
index f835c03..4c7cffa 100644
--- a/src/pybuild_deps/compile_build_dependencies.py
+++ b/src/pybuild_deps/compile_build_dependencies.py
@@ -51,14 +51,14 @@ def resolve(
             log.info("=" * 80)
             log.info(str(ireq))
             log.info("-" * 80)
-            ireq_key = key_from_ireq(ireq)
-            if ireq_key in dependency_cache:
-                all_build_deps.extend(dependency_cache[ireq_key])
+            req_str = str(ireq.req)
+            if req_str in dependency_cache:
+                all_build_deps.extend(dependency_cache[req_str])
                 log.debug(f"{ireq.req} was already solved, moving on...")
                 continue
             build_ireqs = set(self._find_build_dependencies(ireq))
             if not build_ireqs:
-                dependency_cache[ireq_key] = set()
+                dependency_cache[req_str] = set()
                 continue
             try:
                 # Attempt to resolve ireq's transitive dependencies using
@@ -66,7 +66,7 @@ def resolve(
                 # "constraint" that can be used with pip, like when running
                 # "pip install -c constraints.txt some-package"
                 build_dependencies = self._resolve_with_piptools(
-                    package=ireq_key,
+                    package=req_str,
                     ireqs=build_ireqs,
                     constraints=existing_constraints,
                 )
@@ -80,7 +80,7 @@ def resolve(
                 # If this step fails, the same exception will bubble up and explode
                 # in an error.
                 build_dependencies = self._resolve_with_piptools(
-                    package=ireq_key,
+                    package=req_str,
                     ireqs=build_ireqs,
                 )
 
@@ -94,8 +94,11 @@ def resolve(
                     existing_constraints=existing_constraints,
                     dependency_cache=dependency_cache,
                 )
+                build_dependencies = deduplicate_install_requirements(
+                    build_dependencies
+                )
 
-            dependency_cache[ireq_key] = build_dependencies
+            dependency_cache[req_str] = build_dependencies
 
             all_build_deps.extend(build_dependencies)
 
@@ -105,7 +108,7 @@ def _resolve_with_piptools(
         self,
         package: str,
         ireqs: Iterable[InstallRequirement],
-        constraints: set[InstallRequirement],
+        constraints: dict[InstallRequirement] | None = None,
     ) -> set[InstallRequirement]:
         # backup unsafe data before overriding resolver, we will need it later
         # on piptools writer to export the file
@@ -114,7 +117,7 @@ def _resolve_with_piptools(
         # override resolver - we don't want references from other
         self.resolver = BacktrackingResolver(
             constraints=ireqs,
-            existing_constraints=constraints,
+            existing_constraints=constraints or {},
             repository=self.repository,
             allow_unsafe=True,
         )
@@ -148,15 +151,15 @@ def deduplicate_install_requirements(ireqs: Iterable[InstallRequirement]):
     """Deduplicate InstallRequirements."""
     unique_ireqs = {}
     for ireq in ireqs:
-        ireq_key = key_from_ireq(ireq)
-        if ireq_key not in unique_ireqs:
+        req_tuple = ireq.name, get_version(ireq)
+        if req_tuple not in unique_ireqs:
             # NOTE: piptools hacks pip's InstallRequirement to allow support from
             # multiple sources. Let's use the same attr so piptools file writer can
             # use this information.
             # https://github.com/jazzband/pip-tools/blob/53309647980e2a4981db54c0033f98c61142de0b/piptools/resolver.py#L118-L122
             # https://github.com/jazzband/pip-tools/blob/53309647980e2a4981db54c0033f98c61142de0b/piptools/writer.py#L309-L314
             ireq._source_ireqs = getattr(ireq, "_source_ireqs", [ireq])
-            unique_ireqs[ireq_key] = ireq
+            unique_ireqs[req_tuple] = ireq
         else:
-            unique_ireqs[ireq_key]._source_ireqs.append(ireq)
+            unique_ireqs[req_tuple]._source_ireqs.append(ireq)
     return set(unique_ireqs.values())