diff --git a/CHANGES.rst b/CHANGES.rst
index 1ae94d2a4..3e4589fd8 100644
--- a/CHANGES.rst
+++ b/CHANGES.rst
@@ -23,6 +23,13 @@ upgrading your version of coverage.py.
 Unreleased
 ----------
 
+- Fix: in some cases, even with ``[run] relative_files=True``, a data file
+  could be created with absolute path names.  When combined with other relative
+  data files, it was random whether the absolute file names would be made
+  relative or not. If they weren't, then a file would be listed twice in
+  reports, as detailed in `issue 1752`_.  This is now fixed: absolute file
+  names are always made relative when combining.
+
 - Fix: the last case of a match/case statement had an incorrect message if the
   branch was missed.  It said the pattern never matched, when actually the
   branch is missed if the last case always matched.
@@ -33,6 +40,7 @@ Unreleased
   string.  Thanks, `Tanaydin Sirin <pull 1754_>`_.  It is also now documented
   on the :ref:`configuration page <config_report_format>`.
 
+.. _issue 1752: https://github.com/nedbat/coveragepy/issues/1752
 .. _pull 1754: https://github.com/nedbat/coveragepy/pull/1754
 
 
diff --git a/coverage/data.py b/coverage/data.py
index 0db07d156..9513adfca 100644
--- a/coverage/data.py
+++ b/coverage/data.py
@@ -88,7 +88,10 @@ def combinable_files(data_file: str, data_paths: Iterable[str] | None = None) ->
     # We never want to combine those.
     files_to_combine = [fnm for fnm in files_to_combine if not fnm.endswith("-journal")]
 
-    return files_to_combine
+    # Sorting isn't usually needed, since it shouldn't matter what order files
+    # are combined, but sorting makes tests more predictable, and makes
+    # debugging more understandable when things go wrong.
+    return sorted(files_to_combine)
 
 
 def combine_parallel_data(
diff --git a/coverage/files.py b/coverage/files.py
index 71352b8eb..0dd3c4e01 100644
--- a/coverage/files.py
+++ b/coverage/files.py
@@ -489,6 +489,9 @@ def map(self, path: str, exists:Callable[[str], bool] = source_exists) -> str:
 
         # If we get here, no pattern matched.
 
+        if self.relative:
+            path = relative_filename(path)
+
         if self.relative and not isabs_anywhere(path):
             # Auto-generate a pattern to implicitly match relative files
             parts = re.split(r"[/\\]", path)
diff --git a/tests/test_api.py b/tests/test_api.py
index b6ab9cda0..9f65166b9 100644
--- a/tests/test_api.py
+++ b/tests/test_api.py
@@ -1465,3 +1465,23 @@ def test_combine_parallel_data_keep(self) -> None:
         # After combining, the .coverage file & the original combined file should still be there.
         self.assert_exists(".coverage")
         self.assert_file_count(".coverage.*", 2)
+
+    @pytest.mark.parametrize("abs_order, rel_order", [(1, 2), (2, 1)])
+    def test_combine_absolute_then_relative_1752(self, abs_order: int, rel_order: int) -> None:
+        # https://github.com/nedbat/coveragepy/issues/1752
+        # If we're combining a relative data file and an absolute data file,
+        # the absolutes were made relative only if the relative file name was
+        # encountered first.  Test combining in both orders and check that the
+        # absolute file name is properly relative in either order.
+        FILE = "sub/myprog.py"
+        self.make_file(FILE, "a = 1")
+
+        self.make_data_file(suffix=f"{abs_order}.abs", lines={abs_file(FILE): [1]})
+        self.make_data_file(suffix=f"{rel_order}.rel", lines={FILE: [1]})
+
+        self.make_file(".coveragerc", "[run]\nrelative_files = True\n")
+        cov = coverage.Coverage()
+        cov.combine()
+        data = coverage.CoverageData()
+        data.read()
+        assert {os_sep("sub/myprog.py")} == data.measured_files()