Task may subscribe to the same dbrp more than once (Bug?)

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Master at 055e09f

```
kapacitor define o -tick example.tick -dbrp telegraf.autogen -dbrp telegraf.autogen -type stream
~/kap/etc$ kapacitor show o
ID: o
Error:
Template:
Type: stream
Status: disabled
Executing: false
Created: 31 Jul 17 11:15 EDT
Modified: 31 Jul 17 11:15 EDT
LastEnabled: 01 Jan 01 00:00 UTC
Databases Retention Policies: ["telegraf"."autogen" "telegraf"."autogen"]
TICKscript:
var x = 10

var data = stream
|from()
.measurement('cpu')
|alert()
.crit(lambda: TRUE)
.message('hello' + string(x))

DOT:
digraph o {
stream0 -> from1;
from1 -> alert2;
}
```

Start by reproducing the duplicate -dbrp command shown in the issue and tracing how task database-retention-policy subscriptions are handled. The issue does not state whether duplicates should be rejected or deduplicated, so confirm the intended behavior before changing code; done should include coverage for repeated DBRP arguments.

Written by the indexing model from the issue text.