docs: update vmalert and vmagent docs (#6207)

* restore and actualize doc section explaining duplicated labels error
* rm misleading comment about post-aggregation in stream aggregation

(cherry picked from commit e3c226c)

app/vmalert/rule/rule.go

@@ -32,7 +32,7 @@ type Rule interface {
 	close()
 }
 
-var errDuplicate = errors.New("result contains metrics with the same labelset after applying rule labels. See https://docs.victoriametrics.com/vmalert/#series-with-the-same-labelset for details")
+var errDuplicate = errors.New("result contains metrics with the same labelset during evaluation. See https://docs.victoriametrics.com/vmalert/#series-with-the-same-labelset for details")
 
 type ruleState struct {
 	sync.RWMutex

docs/stream-aggregation.md

@@ -598,9 +598,6 @@ some_counter:5m_increase / 5m
 
 This is similar to `rate(some_counter[5m])`.
 
-Please note, opposite to [rate](https://docs.victoriametrics.com/metricsql/#rate), `increase` aggregations can be 
-combined safely afterwards. This is helpful when the aggregation is calculated by more than one vmagent.
-
 Aggregating irregular and sporadic metrics (received from [Lambdas](https://aws.amazon.com/lambda/)
 or [Cloud Functions](https://cloud.google.com/functions)) can be controlled via [staleness_interval](#staleness) option.
 

docs/vmalert.md

@@ -902,6 +902,25 @@ max(vmalert_alerting_rules_last_evaluation_series_fetched) by(group, alertname)
 See more details [here](https://github.com/VictoriaMetrics/VictoriaMetrics/issues/4039).
 This feature is available only if vmalert is using VictoriaMetrics v1.90 or higher as a datasource.
 
+### Series with the same labelset
+
+vmalert can produce the following error message:
+```
+result contains metrics with the same labelset during evaluation
+```
+
+The error means there is a collision between [time series](https://docs.victoriametrics.com/keyConcepts.html#time-series)
+during evaluation.
+
+For example, a rule with `expr: {__name__=~"vmalert_alerts_.*"} > 0` returns two distinct time series in response:
+```
+{__name__="vmalert_alerts_pending",job="vmalert",alertname="HostContextSwitching"} 12
+{__name__="vmalert_alerts_firing",job="vmalert",alertname="HostContextSwitching"} 0
+```
+
+As label `__name__` will be dropped during evaluation, leads to duplicated time series.
+To fix this, one could use function like [label_replace](https://docs.victoriametrics.com/metricsql/#label_replace) to preserve the distinct labelset.
+
 ## mTLS protection
 
 By default `vmalert` accepts http requests at `8880` port (this port can be changed via `-httpListenAddr` command-line flags),