Issue cloudevents#331: Clarify scope of source and id uniqueness

Reword "id" uniqueness description.

Signed-off-by: Alan Conway <aconway@redhat.com>

spec.md

@@ -178,7 +178,7 @@ both appear within the same JSON object.
 
 ### source
 * Type: `URI-reference`
-* Description: This describes the event producer. Often this will include
+* Description: This identifies the event producer. Often this will include
   information such as the type of the event source, the organization
   publishing the event, the process that produced the event, and some unique
   identifiers. The exact syntax and semantics behind the data encoded in the URI
@@ -200,7 +200,9 @@ both appear within the same JSON object.
 * Constraints:
   * REQUIRED
   * MUST be a non-empty string
-  * MUST be unique within the scope of the producer
+  * The producer MUST ensure that source+id is unique for each distinct event.
+  * If a duplicate of a previous event is re-sent it MAY have the same source+id.
+  * A consumer MAY ignore events with the same source+id as a previous event.
 
 ### time
 * Type: `Timestamp`