define ifmeasuring@ only if it doesn't exist yet, issue latex3/tagging-project#920

Author: u-fischer

ChangeLog.txt

@@ -1,3 +1,6 @@
+2025-06-20 Ulrike Fischer
+	* hyperref.dtx: test if \ifmeasuring@ already exists, tagging-project issue #920
+
 2025-06-17 Ulrike Fischer
 	* hyperref.dtx: set the pdfa key automatically if an A-standard is 
       set in \DocumentMetadata. issue #387

hyperref.dtx

@@ -10460,8 +10460,12 @@
 % AMS\LaTeX\ processes all equations twice; we want to make sure
 % that the hyper stuff is not executed twice, so we use the AMS
 % |\ifmeasuring@|, initialized if AMS math is not used.
+% change 2025-06-20: test if the switch already exist, see tagging-project issue 920.
 %    \begin{macrocode}
-\newif\ifmeasuring@\measuring@false
+\ifcsname ifmeasuring@\endcsname  
+\else     
+ \expandafter\newif\csname ifmeasuring@\endcsname\measuring@false
+\fi
 \newcommand\Hy@ifnotmeasuring[1]{#1}
 \AddToHook{package/amsmath/after}
  {