I have the same issue with Graphs on zabbix-2.1.0 (it's possible to create graphs with duplicate names and the same host using API).
Should I create new issue for that?

yes, please do so (unless it's reported already)

Done, ZBX-5868.

A practical way how to reproduce it:

curl -i -X POST -H 'Content-Type: application/json-rpc' -d '{"params": {"rules": {"templateLinkage": {"createMissing": "true"}, "hosts": {"createMissing": "true", "updateExisting": "true"}, "groups": {"createMissing": "true"}}, "source": "<?xml version=\"1.0\" encoding=\"UTF-8\"?><zabbix_export><version>2.0</version><date>2015-09-15T23:41:53Z</date><groups> <group><name>Load Balancer</name> </group></groups><hosts> <host><host>Load Balancer node100</host><name>Load Balancer node100</name><description>satellite Load Balancer: node100 </description><proxy/><status>0</status><ipmi_authtype>-1</ipmi_authtype><ipmi_privilege>2</ipmi_privilege><ipmi_username/><ipmi_password/><groups><group><name>Load Balancer</name></group></groups><interfaces><interface><default>1</default><type>1</type><useip>1</useip><ip>10.33.0.100</ip><dns></dns><port>10050</port><bulk>1</bulk><interface_ref>if1</interface_ref> </interface></interfaces><applications/><items/><discovery_rules/><macros/><inventory/> </host></hosts></zabbix_export>", "format": "xml"}, "jsonrpc": "2.0", "method": "configuration.import","auth": "f45f05ddc5558a7c3a1c72db15162a20", "id": 0}' http://localhost/zabbix/api_jsonrpc.php & \
curl -i -X POST -H 'Content-Type: application/json-rpc' -d '{"params": {"rules": {"templateLinkage": {"createMissing": "true"}, "hosts": {"createMissing": "true", "updateExisting": "true"}, "groups": {"createMissing": "true"}}, "source": "<?xml version=\"1.0\" encoding=\"UTF-8\"?><zabbix_export><version>2.0</version><date>2015-09-15T23:41:53Z</date><groups> <group><name>Load Balancer</name> </group></groups><hosts> <host><host>Load Balancer node101</host><name>Load Balancer node101</name><description>satellite Load Balancer: node101 </description><proxy/><status>0</status><ipmi_authtype>-1</ipmi_authtype><ipmi_privilege>2</ipmi_privilege><ipmi_username/><ipmi_password/><groups><group><name>Load Balancer</name></group></groups><interfaces><interface><default>1</default><type>1</type><useip>1</useip><ip>10.33.0.100</ip><dns></dns><port>10050</port><bulk>1</bulk><interface_ref>if1</interface_ref> </interface></interfaces><applications/><items/><discovery_rules/><macros/><inventory/> </host></hosts></zabbix_export>", "format": "xml"}, "jsonrpc": "2.0", "method": "configuration.import","auth": "f45f05ddc5558a7c3a1c72db15162a20", "id": 0}' http://localhost/zabbix/api_jsonrpc.php

note - replace auth token by yours.

This example creates duplicated host groups.
If you change host name to identical - hosts will be duplicated too.

I've faced with this because I use ansible for a fully automated deploying of zabbix installations with some pre-configuration.
Ansible performs tasks for different targets is parallel, the issue is reproducible in my case for 100%. Using zabbix 3.0.6

ZBX-5705 contains another particular example how to create duplicates.

Similar problem for server side reported in ZBX-11799

This bug results in "Template cannot be linked to another template more than once even through other templates." messages, I've created a patch for 3.0 and 3.2 which show templateid which causes the problem in error message.
Patch is available by name ZBX-5470 in https://github.com/zabbix/zabbix-patches repo, feel free to use it.

How can you prove that duplicated objects caused the error message ?

In my case something impossible by Zabbix logic was done via API, which is a bug and I think it is this very bug (it might be not). I am not reporting the bug or adding any information to it, I just giving away a troubleshooting improvement which might help to locate the problem.

My doubts are directed to the reason why did you get the error message.
I.e. I'm not sure that duplicated objects were the reason. I think that current report is not related to the error you had.

I have the same issue, multiple hosts created with the same name.
None of these hosts can get monitor data in History any more, even after I deleted the duplicated hosts, and only left one host with unique name (zabbix server also restarted).

Multiple entries for ServerActive (localhost and another interface IP) cause duplicated host names creation during agents auto-registration.
Yes, that's misconfiguration, but still.

One more visible sub-issue:
when creating hosts in parallel and linking a template (having triggers or prototypes), it's possible one of those host will get doubled count of triggers from the template, while 2nd host will get 0 triggers inherited.
I.e. on one host triggers/prototypes will be be duplicated: two on a host inherited from single on template.

After discussion with asitals:

How about splitting current status column.

Current implementation

Suggested implementation

Split current status column that mixes host/proxy status and the type of the record into 3 dedicated columns:

• leave status column for host status
• add proxy_status column
• add type column to indicate record type:

Pros:

• prevent duplicates by adding UNIQUE INDEX ON `hosts`(host, type)
• allow disabling a Proxy

Cons:

• host prototypes, there might be a need to allow host prototypes with the same name

Questions:

• Should host prototype be treated as separate type?
• Should Zabbix API continue providing only "status" field (API users will not have to change their scripts)?