Prevent duplicate Prometheus description comments (#2890)

CHANGELOG.md

@@ -16,6 +16,9 @@ release.
 
 ### Metrics
 
+- Handle duplicate description comments during Prometheus conversion.
+  ([#2890](https://github.com/open-telemetry/opentelemetry-specification/pull/2890))
+
 ### Logs
 
 ### Resource

specification/metrics/data-model.md

@@ -1468,6 +1468,14 @@ in keys).
 
 #### Metric Metadata
 
+Prometheus SDK exporters MUST NOT allow duplicate UNIT, HELP, or TYPE
+comments for the same metric name to be returned in a single scrape of the
+Prometheus endpoint. Exporters MUST drop entire metrics to prevent conflicting
+TYPE comments, but SHOULD NOT drop metric points as a result of conflicting
+UNIT or HELP comments. Instead, all but one of the conflicting UNIT and HELP
+comments (but not metric points) SHOULD be dropped. If dropping a comment or
+metric points, the exporter SHOULD warn the user through error logging.
+
 The Name of an OTLP metric MUST be added as the
 [OpenMetrics MetricFamily Name](https://github.com/OpenObservability/OpenMetrics/blob/main/specification/OpenMetrics.md#metricfamily),
 with unit and type suffixes added as described below. The metric name is