Index: generic/tclPathObj.c
==================================================================
--- generic/tclPathObj.c
+++ generic/tclPathObj.c
@@ -1275,10 +1275,19 @@
 {
     FsPath *fsPathPtr;
     Tcl_Obj *pathPtr;
     const char *p;
     int state = 0, count = 0;
+
+    /* if dirPtr is already tclFsPathType, its string representation
+     * may not be canonical.  Try to use its normPathPtr in this case.
+     */
+    if (   (dirPtr->typePtr == &tclFsPathType)
+	&& (PATHOBJ(dirPtr)->cwdPtr == NULL) 
+	&& (PATHOBJ(dirPtr)->normPathPtr != NULL)  ) {
+	dirPtr = PATHOBJ(dirPtr)->normPathPtr;
+    }
 
     /* [Bug 2806250] - this is only a partial solution of the problem.
      * The PATHFLAGS != 0 representation assumes in many places that
      * the "tail" part stored in the normPathPtr field is itself a
      * relative path.  Strings that begin with "~" are not relative paths,

Index: tests/fileSystem.test
==================================================================
--- tests/fileSystem.test
+++ tests/fileSystem.test
@@ -365,10 +365,20 @@
     testPathEqual [file norm /../..] [file norm /]
 } ok
 test filesystem-1.51.1 {file normalisation .. beyond root (Bug 1379287)} {
     testPathEqual [file norm /../../] [file norm /]
 } ok
+test filesystem-1.52 {bug f9f390d0fa: file join where strep is not canonical} -body {
+    set x //foo
+    file normalize $x
+    file join $x bar
+} -result /foo/bar
+test filesystem-1.52.1 {bug f9f390d0fa: file join where strep is not canonical} -body {
+    set x //foo
+    file normalize $x
+    file join $x
+} -result /foo
 
 test filesystem-2.0 {new native path} {unix} {
    foreach f [lsort [glob -nocomplain /usr/bin/c*]] {
        catch {file readlink $f}
    }