Clarify template variable uniqueness

This clarifies that variables in both path and server URL templates
MUST be unique.  This is justified as compatible with our minor
release policy on the following grounds:

* Due to Parameter Object `name` + `in` constraints and the Server
  Object's `variables` field being a map, each variable can only
  map to one paramater or server variable (although the "one"
  paramater might be defined separately yet uniquely for each
  Operation).
* The Path Templating section uses the phrase "Each template
  expression in the path MUST correspond to ***a*** path parameter"
  (emphasis added).
* The Parameter Object uses similar language in the other direction:
  "If in is "path", the name field MUST correspond to ***a***
   template expression".
* The word "a" is interpreted to mean an unknown but unique thing
  in both directions.
* The Server Object's `url` template has always been assumed to
  function in an analogous way to path templating.

Author: handrews

src/oas.md

@@ -62,6 +62,8 @@ sub-delims          = "!" / "$" / "&" / "'" / "(" / ")"
 
 Here, `pchar`, `unreserved`, `pct-encoded` and `sub-delims` definitions are taken from [RFC 3986](https://tools.ietf.org/html/rfc3986). The `path-template` is directly derived from [RFC 3986, section 3.3](https://datatracker.ietf.org/doc/html/rfc3986#section-3.3).
 
+Each template expression MUST NOT appear more than once in a single path template.
+
 See also [Appendix C: Using RFC6570-Based Serialization](#appendix-c-using-rfc6570-based-serialization) for additional guidance.
 
 ### Media Types
@@ -596,6 +598,8 @@ iprivate       =  %xE000-F8FF / %xF0000-FFFFD / %x100000-10FFFD
 
 Here, `literals`, `pct-encoded`, `ucschar` and `iprivate` definitions are taken from [RFC 6570](https://www.rfc-editor.org/rfc/rfc6570), incorporating the corrections specified in [Errata 6937](https://www.rfc-editor.org/errata/eid6937) for `literals`.
 
+Each server variable MUST NOT appear more than once in the URL template.
+
 See the [Paths Object](#paths-object) for guidance on constructing full request URLs.
 
 ##### Fixed Fields
@@ -967,7 +971,7 @@ These fields MAY be used with either `content` or `schema`.
 
 | Field Name | Type | Description |
 | ---- | :----: | ---- |
-| <a name="parameter-name"></a>name | `string` | **REQUIRED**. The name of the parameter. Parameter names are _case sensitive_. <ul><li>If [`in`](#parameter-in) is `"path"`, the `name` field MUST correspond to a template expression occurring within the [path](#paths-path) field in the [Paths Object](#paths-object). See [Path Templating](#path-templating) for further information.<li>If [`in`](#parameter-in) is `"header"` and the `name` field is `"Accept"`, `"Content-Type"` or `"Authorization"`, the parameter definition SHALL be ignored.<li>If `in` is `"querystring"`, or for [certain combinations](#style-examples) of [`style`](#parameter-style) and [`explode`](#parameter-explode), the value of `name` is not used in the parameter serialization.<li>For all other cases, the `name` corresponds to the parameter name used by the [`in`](#parameter-in) field.</ul> |
+| <a name="parameter-name"></a>name | `string` | **REQUIRED**. The name of the parameter. Parameter names are _case sensitive_. <ul><li>If [`in`](#parameter-in) is `"path"`, the `name` field MUST correspond to a single template expression occurring within the [path](#paths-path) field in the [Paths Object](#paths-object). See [Path Templating](#path-templating) for further information.<li>If [`in`](#parameter-in) is `"header"` and the `name` field is `"Accept"`, `"Content-Type"` or `"Authorization"`, the parameter definition SHALL be ignored.<li>If `in` is `"querystring"`, or for [certain combinations](#style-examples) of [`style`](#parameter-style) and [`explode`](#parameter-explode), the value of `name` is not used in the parameter serialization.<li>For all other cases, the `name` corresponds to the parameter name used by the [`in`](#parameter-in) field.</ul> |
 | <a name="parameter-in"></a>in | `string` | **REQUIRED**. The ___location of the parameter. Possible values are `"query"`, `"querystring"`, `"header"`, `"path"` or `"cookie"`. |
 | <a name="parameter-description"></a>description | `string` | A brief description of the parameter. This could contain examples of use. [CommonMark syntax](https://spec.commonmark.org/) MAY be used for rich text representation. |
 | <a name="parameter-required"></a>required | `boolean` | Determines whether this parameter is mandatory. If the [parameter ___location](#parameter-in) is `"path"`, this field is **REQUIRED** and its value MUST be `true`. Otherwise, the field MAY be included and its default value is `false`. |