Export to library offset due to double episode

*I'm submitting a ...*
- [x] bug report
- [ ] feature request
- [ ] support request

## General infomration

### Prerequisites

* [x] Are you running the latest version?

### Description

When exporting a show to the library double episodes are not handled correctly. If the first two episodes are a double episode, episode 3 is exported as episode 2, causing an offset between the metadata and the stream url.

### Steps to Reproduce

1. Export a show (ie Star Trek Voyager)
2. Play episode 2 (ep1 part 2)
3. Episode 3 starts playing instead

**Expected behavior:** ep1+2 should be exported as S01E01-02.strm with matching metadata, ep3 should be exported as S01E03.strm with matching metadata.

**Actual behavior:** ep1+2 is exported as S01E01.strm, metadata matches ep1 only. Ep3 is exported as ep2 with metadata for ep2, ep4 is exported as ep3 and so on.

## Context (Environment)

### Installation

* [x] I installed the plugin via zip from the Releases page
* [ ] I´m using the Netflix Repo
* [ ] I´m using a different source (Please tell which)

### Operating System

* [ ] Linux (x86/x64)
* [ ] OSX (x86/x64)
* [ ] Windows (x86/x64)
* [x] Linux (ARM)
* [ ] Android

#### Additional informatin on the environment

Running osmc latest nightly on rpi 3B.

Thanks!