nit: does EmitTag contract include not emitting a tag with a key that is already in the tags slice?

Description of the bug

nit: EmitTag should either be documented as requiring as pre-condition that the key argument not already be in the c.tags slice, or enforce uniqueness and generate an error

https://github.com/oasislabs/ekiden/blob/221f43394d4469bd71ab72302feec15f67c0944e/go/tendermint/abci/context.go#L86

EmitTag paired with GetTag feel inconsistent: one can emit duplicate tag entries, and not be able to find any but the first entry.

Steps to reproduce
1.

Expected behavior

Either EmitTag includes as pre-condition that keys are unique in the history of key/value pairs entered as tags to a context, or GetTag should have the ability to find all tag entries with a given key rather than just the first entry.

Environment

• OS:
• Versions of relevant software and tools:

Additional information
Anything else you think would be useful; including context and suggested fixes.

Read go/tendermint/abci/context.go at the linked EmitTag implementation, then compare its behavior with GetTag. Determine which tag-key contract the API should document or enforce, and verify that the chosen behavior consistently handles repeated keys.