AZW3 Output: Remove duplicate anchors to workaround some Kindle renderers using the last occurrence of an anchor as the target instead of the first. Fixes #1454199 [hyperlink conversion to azw3 and display on kindle](https://bugs.launchpad.net/calibre/+bug/1454199)

src/calibre/ebooks/conversion/plugins/mobi_output.py

@@ -191,6 +191,8 @@ class MOBIOutput(OutputFormatPlugin):
         self.check_for_periodical()
 
         if create_kf8:
+            from calibre.ebooks.mobi.writer8.cleanup import remove_duplicate_anchors
+            remove_duplicate_anchors(self.oeb)
             # Split on pagebreaks so that the resulting KF8 is faster to load
             from calibre.ebooks.oeb.transforms.split import Split
             Split()(self.oeb, self.opts)
@@ -301,10 +303,12 @@ class AZW3Output(OutputFormatPlugin):
     def convert(self, oeb, output_path, input_plugin, opts, log):
         from calibre.ebooks.mobi.writer2.resources import Resources
         from calibre.ebooks.mobi.writer8.main import create_kf8_book
+        from calibre.ebooks.mobi.writer8.cleanup import remove_duplicate_anchors
 
         self.oeb, self.opts, self.log = oeb, opts, log
         opts.mobi_periodical = self.is_periodical
         passthrough = getattr(opts, 'mobi_passthrough', False)
+        remove_duplicate_anchors(oeb)
 
         resources = Resources(self.oeb, self.opts, self.is_periodical,
                 add_fonts=True, process_images=False)

src/calibre/ebooks/mobi/writer8/cleanup.py

@@ -15,7 +15,8 @@ class CSSCleanup(object):
         self.log, self.opts = log, opts
 
     def __call__(self, item, stylizer):
-        if not hasattr(item.data, 'xpath'): return
+        if not hasattr(item.data, 'xpath'):
+            return
 
         # The Kindle touch displays all black pages if the height is set on
         # body
@@ -23,3 +24,19 @@ class CSSCleanup(object):
             style = stylizer.style(body)
             style.drop('height')
 
+def remove_duplicate_anchors(oeb):
+    # The Kindle apparently has incorrect behavior for duplicate anchors, see
+    # https://bugs.launchpad.net/calibre/+bug/1454199
+    for item in oeb.spine:
+        if not hasattr(item.data, 'xpath'):
+            continue
+        seen = set()
+        for tag in item.data.xpath('//*[@id or @name]'):
+            for attr in ('id', 'name'):
+                anchor = tag.get(attr)
+                if anchor is not None:
+                    if anchor in seen:
+                        oeb.log.debug('Removing duplicate anchor:', anchor)
+                        tag.attrib.pop(attr)
+                    else:
+                        seen.add(anchor)