Double checked if it's intentional - it's also possible in single host update. There's only an error if you add an tag with same tag/value.

So this might be more a discussion if it's good practice to allow multiple same tags than an "Problem report". So here's my view:

Commonly tags are seen as key/value pairs and keys are defined as unique identifiers. So I would say this route should be followed by Zabbix too. It also may come to misbehaviour in queries, event correlations or maintenance plans.

Currently it is possible yes and this is how it was designed to work

As mbuhholcs said, this is an expected behavior for tags in Zabbix. We allow tags with same name, but different values (or no value at all). It is only not allowed to have multiple instances of same tag:value pair.

But although it works as designed, it is not properly documented. This behavior should be explained in Event tags page.

I see and perfectly understand. To be true, I tested the behaviour with Maintenance windows (which is also not intuitive at first) and actually can see some benefits. But it should be documented - it may even need an short text on frontend?

Documentation updated:

• Event tags in 4.4, 5.0
• Templates - > Mass update in 4.4, 5.0
• Hosts -> Mass update in 4.4, 5.0
• Triggers -> Mass update in 4.4, 5.0
• Configuring a trigger in 4.4, 5.0