Prohibit duplicate collection identifiers and pattern variables

Author: jskeet

aip/general/0122.md

@@ -78,6 +78,9 @@ form of the noun used for the resource. (For example, a collection of
     and plural terms are the same ("moose"), the non-pluralized (singular) form
     is correct. Collection segments **must not** "coin" words by adding "s" in
     such cases (e.g, avoid "infos").
+- Within any given single resource name, collection identifiers **must** be
+  unique. (e.g. `people/xyz/people/abc` is invalid)
+
 
 #### Nested collections
 
@@ -354,6 +357,7 @@ isolation of logical concerns per-resource.
 
 ## Changelog
 
+- **2023-09-19**: Prohibit duplicate collection identifiers.
 - **2023-09-01**: Add a clause that allows embedding for revision resource
   messages.
 - **2023-08-10**: Explicitly disallow embedding resource messages in a resource.

aip/general/0123.md

@@ -75,6 +75,9 @@ message Topic {
 - Pattern variables (the segments within braces) **must** use `snake_case`, and
   **must not** use an `_id` suffix.
 - Pattern variables **must** conform to the format `[a-z][_a-z0-9]*[a-z0-9]`.
+- Pattern variables **must** be unique within any given pattern. (e.g.
+  `projects/{abc}/topics/{abc}` is invalid; this is usually a natural
+  corollary of collection identifiers being unique within a pattern.)
 - Singular **must** be the lower camel case of the type.
   - Pattern variables **must** be the singular form of the resource type e.g.
     a pattern variable representing a `Topic` resource ID is named `{topic}`.
@@ -116,6 +119,7 @@ such as UpperCamelCase and snake_case.
 
 ## Changelog
 
+- **2023-09-19**: Prohibit duplicate pattern variables.
 - **2023-05-06**: Adding requirement of singular and plural.
 - **2023-01-28**: Clarifying guidance for the resource type name.
 - **2022-10-28**: Added pattern variable format guidance.