Merging of entities should sanitize from_entity_id input

[mdgreenfield]

Describe the bug
When two entities are being merged, duplicate from_entity_ids results in duplicate entity aliases in the "to" entity.

To Reproduce
Steps to reproduce the behavior:

1. Add an auth mount - vault auth enable -path=userpass1 userpass
2. Get the auth mount accessor - mount1accessor=$(vault auth list -format=json | jq -r '.["userpass1/"].accessor')
3. Create an entity - originalParentID=$(vault write -field=id -force identity/entity)
4. Create an entity alias - vault write -field=id identity/entity-alias name=foobar canonical_id=$originalParentID mount_accessor=$mount1accessor
5. Create a second entity - newParentID=$(vault write -field=id identity/entity name=foobar)
6. Merge duplicate originalParentIDs into newParentID - vault write identity/entity/merge to_entity_id=$newParentID from_entity_ids=$originalParentID,$originalParentID
7. Observe that the number of entity aliases on newParentID is incorrect. It should be 1 but it is 2. - vault read -format=json identity/entity/id/$newParentID | jq '.data.aliases | length'

Expected behavior
If from_entity_ids contains duplicates when merging two entities, they should be de-duplicated.

Environment:

• Vault Server Version (retrieve with vault status): 1.5.4
• Vault CLI Version (retrieve with vault version): 1.5.4