diff --git a/sbom.py b/sbom.py
index c7122139..9a3cfa00 100644
--- a/sbom.py
+++ b/sbom.py
@@ -24,8 +24,9 @@
 import tarfile
 import typing
 import zipfile
+from functools import cache
 from pathlib import Path
-from typing import Any, NotRequired, TypedDict, cast
+from typing import Any, LiteralString, NotRequired, TypedDict, cast
 from urllib.request import urlopen
 
 
@@ -90,9 +91,20 @@ class CreationInfo(TypedDict):
     licenseListVersion: str
 
 
-def spdx_id(value: str) -> str:
+# Cache of values that we've seen already. We use this
+# to de-duplicate values and their corresponding SPDX ID.
+_SPDX_IDS_TO_VALUES: dict[str, Any] = {}
+
+
+@cache
+def spdx_id(value: LiteralString) -> str:
     """Encode a value into characters that are valid in an SPDX ID"""
-    return re.sub(r"[^a-zA-Z0-9.\-]+", "-", value)
+    value_as_spdx_id = re.sub(r"[^a-zA-Z0-9.\-]+", "-", value)
+    # To avoid collisions we append a hash suffix.
+    suffix = hashlib.sha256(value.encode()).hexdigest()[:8]
+    value_as_spdx_id = f"{value_as_spdx_id}-{suffix}"
+    assert _SPDX_IDS_TO_VALUES.setdefault(value_as_spdx_id, value) == value
+    return value_as_spdx_id
 
 
 def calculate_package_verification_codes(sbom: SBOM) -> None:
diff --git a/tests/sbom/sbom-with-pip-removed.json b/tests/sbom/sbom-with-pip-removed.json
index 0ba2d4c0..b2646f23 100644
--- a/tests/sbom/sbom-with-pip-removed.json
+++ b/tests/sbom/sbom-with-pip-removed.json
@@ -13,9 +13,9 @@
   "packages": [],
   "relationships": [
     {
-      "relatedSpdxElement": "SPDXRef-FILE-Modules-expat-COPYING",
+      "relatedSpdxElement": "SPDXRef-FILE-Modules-expat-COPYING-497fb0c3",
       "relationshipType": "CONTAINS",
-      "spdxElementId": "SPDXRef-PACKAGE-expat"
+      "spdxElementId": "SPDXRef-PACKAGE-expat-83b93528"
     }
   ]
 }
diff --git a/tests/sbom/sbom-with-pip.json b/tests/sbom/sbom-with-pip.json
index e758c182..d2ee1c6f 100644
--- a/tests/sbom/sbom-with-pip.json
+++ b/tests/sbom/sbom-with-pip.json
@@ -12,7 +12,7 @@
   "files": [],
   "packages": [
     {
-      "SPDXID": "SPDXRef-PACKAGE-pip",
+      "SPDXID": "SPDXRef-PACKAGE-pip-ced959c1",
       "name": "pip",
       "versionInfo": "24.0",
       "licenseConcluded": "MIT",
@@ -38,19 +38,19 @@
   ],
   "relationships": [
     {
-      "relatedSpdxElement": "SPDXRef-FILE-Modules-expat-COPYING",
+      "relatedSpdxElement": "SPDXRef-FILE-Modules-expat-COPYING-497fb0c3",
       "relationshipType": "CONTAINS",
-      "spdxElementId": "SPDXRef-PACKAGE-expat"
+      "spdxElementId": "SPDXRef-PACKAGE-expat-83b93528"
     },
     {
-      "relatedSpdxElement": "SPDXRef-PACKAGE-urllib3",
+      "relatedSpdxElement": "SPDXRef-PACKAGE-urllib3-b7a198af",
       "relationshipType": "DEPENDS_ON",
-      "spdxElementId": "SPDXRef-PACKAGE-pip"
+      "spdxElementId": "SPDXRef-PACKAGE-pip-ced959c1"
     },
     {
-      "relatedSpdxElement": "SPDXRef-PACKAGE-pip",
+      "relatedSpdxElement": "SPDXRef-PACKAGE-pip-ced959c1",
       "relationshipType": "DEPENDS_ON",
-      "spdxElementId": "SPDXRef-PACKAGE-cpython"
+      "spdxElementId": "SPDXRef-PACKAGE-cpython-608f998c"
     }
   ]
 }
diff --git a/tests/test_sbom.py b/tests/test_sbom.py
index 45e66041..8e7713dc 100644
--- a/tests/test_sbom.py
+++ b/tests/test_sbom.py
@@ -11,6 +11,22 @@
 import sbom
 
 
+@pytest.mark.parametrize(
+    ["value", "expected"],
+    [
+        ("abc", "abc-ba7816bf"),
+        ("def", "def-cb8379ac"),
+        ("SPDXRef-PACKAGE-pip", "SPDXRef-PACKAGE-pip-ced959c1"),
+        ("SPDXRef-PACKAGE-cpython", "SPDXRef-PACKAGE-cpython-79ab18d2"),
+        ("SPDXRef-PACKAGE-urllib3", "SPDXRef-PACKAGE-urllib3-b8ab4751"),
+    ],
+)
+def test_spdx_id(value: str, expected: str) -> None:
+    assert sbom.spdx_id(value) == expected
+    # Check we get the same value next time
+    assert sbom.spdx_id(value) == expected
+
+
 @pytest.mark.parametrize(
     ["package_sha1s", "package_verification_code"],
     [