Initial attempt at allowing multiple references to the same footnote

Previously this generated the same ``id`` attribute for every link in rich text for the same footnote, which breaks the "Back to content" link.
This generates a unique ``id`` for each footnote reference.
This commit does not expose these links on the front-end (yet), but at least the "back" link will point to a single id.

wagtail_footnotes/blocks.py

@@ -1,3 +1,4 @@
+from collections import defaultdict
 import re
 
 from django.core.exceptions import ValidationError
@@ -23,6 +24,7 @@ def __init__(self, **kwargs):
             self.features = []
         if "footnotes" not in self.features:
             self.features.append("footnotes")
+        self.footnotes = {}
 
     def replace_footnote_tags(self, value, html, context=None):
         if context is None:
@@ -36,15 +38,20 @@ def replace_footnote_tags(self, value, html, context=None):
         page = new_context["page"]
         if not hasattr(page, "footnotes_list"):
             page.footnotes_list = []
+        if not hasattr(page, "footnotes_references"):
+            page.footnotes_references = defaultdict(list)
         self.footnotes = {footnote.uuid: footnote for footnote in page.footnotes.all()}
 
         def replace_tag(match):
+            footnote_uuid = match.group(1)
             try:
-                index = self.process_footnote(match.group(1), page)
+                index = self.process_footnote(footnote_uuid, page)
             except (KeyError, ValidationError):
                 return ""
             else:
-                return f'<a href="#footnote-{index}" id="footnote-source-{index}"><sup>[{index}]</sup></a>'
+                link_id = f'footnote-source-{index}-{len(page.footnotes_references[footnote_uuid])}'
+                page.footnotes_references[footnote_uuid].append(link_id)
+                return f'<sup id="{link_id}"><a href="#footnote-{index}">[{index}]</a></sup>'
 
         return mark_safe(FIND_FOOTNOTE_TAG.sub(replace_tag, html))
 
@@ -57,7 +64,6 @@ def render(self, value, context=None):
 
     def render_basic(self, value, context=None):
         html = super().render_basic(value, context)
-
         return self.replace_footnote_tags(value, html, context=context)
 
     def process_footnote(self, footnote_id, page):