diff --git a/doc/Guidebook.mn b/doc/Guidebook.mn
index 319f31502..67ad5dbb6 100644
--- a/doc/Guidebook.mn
+++ b/doc/Guidebook.mn
@@ -31,7 +31,8 @@
 .lt 70n
 .\}
 .
-.so tmac.nh	\" extra macros which aren't in tmac.n
+.so tmac.nh \" extra macros which aren't in tmac.n
+.if !\n(nH .so doc/tmac.nh
 .
 .\" building Guidebook.txt doesn't have CR font available; groff 1.23 issues
 .\" a warning each time any font can't be loaded; earlier versions silently
diff --git a/doc/tmac.nh b/doc/tmac.nh
index 1a7ae1231..d4dd18a11 100644
--- a/doc/tmac.nh
+++ b/doc/tmac.nh
@@ -8,6 +8,10 @@
 .\"   cluttered as their number increased.  It now uses the '.so' directive
 .\"   to include this file.  (tmac.n is passed to 'roff on the command line.)
 .
+.\" Protect against being sourced twice.
+.nr nH +1
+.if \n(nH>1 .nx
+.
 .\" labeled paragraph start
 .\" .PS word
 .\"	set the width for the label column