Adding "wait" tag duplicates previous tags

Hi,

when I set task's tags via `task mod tag:"blah,wait"`, the tags typed before "wait" are duplicated. Tags after "wait" are correct and modifying tags via `task mod +blah +wait` works as expected.

This actually affects all tags which start with "wait", so submitting e.g. `task mod tag:blah,waitfooofofofo` has the same effect.

* What command(s) did you run?

Below commands reproduce the issue

```
$ task add abc
Created task 60.

$ task 60 mod tags:"foo,wait"
Modifying task 60 'abc'.
Modified 1 task.

$ task _get 60.tags
foo,foo,wait

$ task 60 mod tags:"foo,bar,wait
$ task _get 60.tags
foo,bar,foo,bar,wait

$ task 60 mod tags:"foo,bar,wait,other"
$ task _get 60.tags
foo,bar,foo,bar,wait,other
```

* What did you expect to happen?

When modifying task via e.g. `task 60 mod tags:"foo,wait"` I expected that only "foo" and "bar" tasks will appear, foo not being duplicated.

* Paste the output of the ```task diag``` command.

```
task 2.5.1
Platform: Linux

Compiler
Version: 6.3.0 20170221
Caps: +stdc +stdc_hosted +LP64 +c8 +i32 +l64 +vp64 +time_t64
Compliance: C++11

Build Features
CMake: 3.7.2
libuuid: libuuid + uuid_unparse_lower
libgnutls: 3.5.8
Build type: None

Configuration
File: /home/mgoral/.taskrc (found), 2494 bytes, mode 100644
Data: /home/mgoral/.task (found), dir, mode 40755
Locking: Enabled
GC: Enabled
$EDITOR: nvim
Server: mgoral.org:53589
CA: ~/sync/taskd-certs/ca.cert.pem, readable, 3731 bytes
Trust: strict
Certificate: ~/sync/taskd-certs/client.cert.pem, readable, 3714 bytes
Key: ~/sync/taskd-certs/client.key.pem, readable, 25303 bytes
Ciphers: NORMAL
Creds: Public/virgo/************************************

Hooks
System: Enabled
Location: /home/mgoral/.task/hooks
(-none-)

Tests
$TERM: screen-256color (223x107)
Dups: Scanned 591 tasks for duplicate UUIDs:
No duplicates found
Broken ref: Scanned 591 tasks for broken references:
No broken references found
```