diff --git a/cwltool/pack.py b/cwltool/pack.py
index 898f5485b..aab64e3ac 100644
--- a/cwltool/pack.py
+++ b/cwltool/pack.py
@@ -1,9 +1,11 @@
 from __future__ import absolute_import
 import copy
+import re
 from typing import Any, Callable, Dict, List, Set, Text, Union, cast
 
 from schema_salad.ref_resolver import Loader
 from six.moves import urllib
+from ruamel.yaml.comments import CommentedSeq, CommentedMap
 
 from .process import shortname, uniquename
 import six
@@ -64,7 +66,12 @@ def replace_refs(d, rewrite, stem, newstem):
                 if v in rewrite:
                     d[s] = rewrite[v]
                 elif v.startswith(stem):
-                    d[s] = newstem + v[len(stem):]
+                    id_ = v[len(stem):]
+                    # prevent appending newstems if tool is already packed
+                    if id_.startswith(newstem.strip("#")):
+                        d[s] = "#" + id_
+                    else:
+                        d[s] = newstem + id_
             replace_refs(v, rewrite, stem, newstem)
 
 def import_embed(d, seen):
@@ -106,12 +113,16 @@ def loadref(b, u):
 
     mainpath, _ = urllib.parse.urldefrag(uri)
 
-    def rewrite_id(r, mainuri):
-        # type: (Text, Text) -> None
+    def rewrite_id(r, mainuri, document_packed=False):
+        # type: (Text, Text, bool) -> None
         if r == mainuri:
             rewrite[r] = "#main"
         elif r.startswith(mainuri) and r[len(mainuri)] in ("#", "/"):
-            pass
+            if document_packed:
+                # rewrite tool and mainuri ids in a packed document
+                tool_id = re.search("#[^/]*$", r)
+                if tool_id:
+                    rewrite[r] = tool_id.group()
         else:
             path, frag = urllib.parse.urldefrag(r)
             if path == mainpath:
@@ -122,9 +133,10 @@ def rewrite_id(r, mainuri):
 
     sortedids = sorted(ids)
 
+    is_document_packed = all(id.startswith(uri) for id in sortedids)
     for r in sortedids:
         if r in document_loader.idx:
-            rewrite_id(r, uri)
+            rewrite_id(r, uri, is_document_packed)
 
     packed = {"$graph": [], "cwlVersion": metadata["cwlVersion"]
               }  # type: Dict[Text, Any]
@@ -132,6 +144,9 @@ def rewrite_id(r, mainuri):
     schemas = set()  # type: Set[Text]
     for r in sorted(runs):
         dcr, metadata = document_loader.resolve_ref(r)
+        if isinstance(dcr, CommentedSeq):
+            dcr = dcr[0]
+            dcr = cast(CommentedMap, dcr)
         if not isinstance(dcr, dict):
             continue
         for doc in (dcr, metadata):
@@ -141,7 +156,7 @@ def rewrite_id(r, mainuri):
         if dcr.get("class") not in ("Workflow", "CommandLineTool", "ExpressionTool"):
             continue
         dc = cast(Dict[Text, Any], copy.deepcopy(dcr))
-        v = rewrite[r]
+        v = rewrite.get(r, r + "#main")
         dc["id"] = v
         for n in ("name", "cwlVersion", "$namespaces", "$schemas"):
             if n in dc:
diff --git a/tests/test_pack.py b/tests/test_pack.py
index 34a14e6d3..ebddd79eb 100644
--- a/tests/test_pack.py
+++ b/tests/test_pack.py
@@ -60,3 +60,32 @@ def test_pack_missing_cwlVersion(self):
         packed = json.loads(print_pack(document_loader, processobj, uri, metadata))
 
         self.assertEqual('v1.0', packed["cwlVersion"])
+
+    def test_pack_idempotence_tool(self):
+        """Test to ensure that pack produces exactly the same document for
+           an already packed document"""
+
+        # Testing single tool
+        self._pack_idempotently("tests/wf/hello_single_tool.cwl")
+
+    def test_pack_idempotence_workflow(self):
+        """Test to ensure that pack produces exactly the same document for
+           an already packed document"""
+
+        # Testing workflow
+        self._pack_idempotently("tests/wf/count-lines1-wf.cwl")
+
+    def _pack_idempotently(self, document):
+        self.maxDiff = None
+        document_loader, workflowobj, uri = fetch_document(
+            get_data(document))
+        document_loader, avsc_names, processobj, metadata, uri = validate_document(
+            document_loader, workflowobj, uri)
+        # generate pack output dict
+        packed = json.loads(print_pack(document_loader, processobj, uri, metadata))
+
+        document_loader, workflowobj, uri2 = fetch_document(packed)
+        document_loader, avsc_names, processobj, metadata, uri2 = validate_document(
+            document_loader, workflowobj, uri)
+        double_packed = json.loads(print_pack(document_loader, processobj, uri2, metadata))
+        self.assertEqual(packed, double_packed)
diff --git a/tests/wf/count-lines1-wf.cwl b/tests/wf/count-lines1-wf.cwl
new file mode 100644
index 000000000..77cbf3aa0
--- /dev/null
+++ b/tests/wf/count-lines1-wf.cwl
@@ -0,0 +1,25 @@
+#!/usr/bin/env cwl-runner
+class: Workflow
+cwlVersion: v1.0
+
+inputs:
+  file1:
+    type: File
+
+outputs:
+  count_output:
+    type: int
+    outputSource: step2/output
+
+steps:
+  step1:
+    run: wc-tool.cwl
+    in:
+      file1: file1
+    out: [output]
+
+  step2:
+    run: parseInt-tool.cwl
+    in:
+      file1: step1/output
+    out: [output]
diff --git a/tests/wf/parseInt-tool.cwl b/tests/wf/parseInt-tool.cwl
new file mode 100644
index 000000000..42f166bde
--- /dev/null
+++ b/tests/wf/parseInt-tool.cwl
@@ -0,0 +1,16 @@
+#!/usr/bin/env cwl-runner
+
+class: ExpressionTool
+requirements:
+  - class: InlineJavascriptRequirement
+cwlVersion: v1.0
+
+inputs:
+  file1:
+    type: File
+    inputBinding: { loadContents: true }
+
+outputs:
+  output: int
+
+expression: "$({'output': parseInt(inputs.file1.contents)})"
diff --git a/tests/wf/wc-tool.cwl b/tests/wf/wc-tool.cwl
new file mode 100644
index 000000000..165585494
--- /dev/null
+++ b/tests/wf/wc-tool.cwl
@@ -0,0 +1,17 @@
+#!/usr/bin/env cwl-runner
+
+class: CommandLineTool
+cwlVersion: v1.0
+
+inputs:
+  file1: File
+
+outputs:
+  output:
+    type: File
+    outputBinding: { glob: output }
+
+baseCommand: [wc, -l]
+
+stdin: $(inputs.file1.path)
+stdout: output