zsh: prevent duplicate default REMAINDER spec in generated completions

When completing commands with a subparser that uses nargs=REMAINDER, zsh could
emit:

    _arguments:comparguments:327: doubled rest argument definition: *::: :->repro

Root cause:

- The zsh template appended the default positional/rest specs (`': :{prefix}_commands'` and
  `'*::: :->{name}'`) on every function invocation. It only checked for generic
  variadic/remainder tokens (`(*)`, `(-)*`) and not whether the exact default entry
  (`*::: :->{name}`) had already been added. Repeated invocations led to duplicated
  rest specs and the `_arguments` error above.

Fix:

- Add a per-function guard variable `{prefix}_defaults_added`, initialized alongside
  `{prefix}_options` and set to `1` after the first append. This guarantees that the
  default `'*::: :->{name}'` is added at most once per session.
- Keep (and extend) the presence checks: only append the defaults if neither a
  variadic/rest token (`(*)`, `(-)*`) nor the exact default entry is already present.

Impact:

- Eliminates duplicated `*::: :->…` entries and the “doubled rest argument definition”
  error in zsh, including across multiple invocations and after re-sourcing the file.
- Generated zsh output is unchanged except for the guard variable lines.

Author: ThomasWaldmann

shtab/__init__.py

@@ -606,10 +606,14 @@ def command_case(prefix, options):
 
         return f"""\
 {prefix}() {{
-  local context state line curcontext="$curcontext" one_or_more='(*)' remainder='(-)*'
+  local context state line curcontext="$curcontext" one_or_more='(*)' remainder='(-)*' default='*::: :->{name}'
 
-  if ((${{{prefix}_options[(I)${{(q)one_or_more}}*]}} + ${{{prefix}_options[(I)${{(q)remainder}}*]}} == 0)); then  # noqa: E501
-    {prefix}_options+=(': :{prefix}_commands' '*::: :->{name}')
+  # Add default positional/remainder specs only if none exist, and only once per session
+  if (( ! {prefix}_defaults_added )); then
+    if (( ${{{prefix}_options[(I)${{(q)one_or_more}}*]}} + ${{{prefix}_options[(I)${{(q)remainder}}*]}} + ${{{prefix}_options[(I)${{(q)default}}]}} == 0 )); then
+      {prefix}_options+=(': :{prefix}_commands' '*::: :->{name}')
+    fi
+    {prefix}_defaults_added=1
   fi
   _arguments -C -s ${prefix}_options
 
@@ -631,6 +635,9 @@ def command_option(prefix, options):
 {prefix}_options=(
   {arguments}
 )
+
+# guard to ensure default positional specs are added only once per session
+{prefix}_defaults_added=0
 """
 
     def command_list(prefix, options):