patch 8.2.4942: error when setting 'filetype' in help file again

Problem:    Error when setting 'filetype' in help file again.
Solution:   Deal with text property type already existing. (closes #10409)

runtime/import/dist/vimhelp.vim

@@ -9,11 +9,17 @@ export def HighlightGroups()
   while getline(lnum) !~ '===' && lnum < line('$')
     var word: string = getline(lnum)->matchstr('^\w\+\ze\t')
     if word->hlexists()
-      prop_type_add('help-hl-' .. word, {
-	bufnr: buf,
-	highlight: word,
-	combine: false,
-	})
+      var name = 'help-hl-' .. word
+      if prop_type_list({bufnr: buf})->match(name) == -1
+	prop_type_add('help-hl-' .. word, {
+	  bufnr: buf,
+	  highlight: word,
+	  combine: false,
+	  })
+      else
+	# was called before, delete existing properties
+	prop_remove({type: name, bufnr: buf})
+      endif
       prop_add(lnum, 1, {length: word->strlen(), type: 'help-hl-' .. word})
     endif
     ++lnum

src/version.c

@@ -746,6 +746,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    4942,
 /**/
     4941,
 /**/