Sorting out redundant text from screen scraper application You are writing a screen scraper application monitoring a text-only chat window. Text is added at the bottom of the window. 

The application takes screenshot of the chat window. If a change has occurred since last screenshot (new_screenshot != old_screenshot), the screenshot is saved. 

After X time, all images are merged to one image, where the oldest image is on the top. This large image is send to a server for OCR, and a string of text is returned. 

Challenge: Sort out redundant text.

Example: Chat window is 5 lines high and is initially empty. The solution must work with empty and not-empty initial chat window. More than one line can be added at each screenshot.

Input to algorithm:

1 Lorem ipsum dolor sit amet, 
1 Lorem ipsum dolor sit amet,
2 consectetur adipiscing elit
1 Lorem ipsum dolor sit amet,
2 consectetur adipiscing elit
3 Mauris porttitor enim sed tincidunt interdum.
1 Lorem ipsum dolor sit amet,
2 consectetur adipiscing elit
3 Mauris porttitor enim sed tincidunt interdum.
4 Morbi elementum erat nec nulla auctor, eget porta odio aliquet.
1 Lorem ipsum dolor sit amet,
2 consectetur adipiscing elit
3 Mauris porttitor enim sed tincidunt interdum.
4 Morbi elementum erat nec nulla auctor, eget porta odio aliquet.
5 Nam aliquet velit vel elementum tristique.
2 consectetur adipiscing elit 
3 Mauris porttitor enim sed tincidunt interdum. 
4 Morbi elementum erat nec nulla auctor, eget porta odio aliquet. 
5 Nam aliquet velit vel elementum tristique. 
6 Donec ac tincidunt urna. 
3 Mauris porttitor enim sed tincidunt interdum.
4 Morbi elementum erat nec nulla auctor, eget porta odio aliquet.
5 Nam aliquet velit vel elementum tristique.
6 Donec ac tincidunt urna.
7 Proin pretium, metus non porttitor lobortis, tortor sem rhoncus urna
4 Morbi elementum erat nec nulla auctor, eget porta odio aliquet.
5 Nam aliquet velit vel elementum tristique.
6 Donec ac tincidunt urna.
7 Proin pretium, metus non porttitor lobortis, tortor sem rhoncus urna
8 quis finibus leo lorem sed lacus.
5 Nam aliquet velit vel elementum tristique.
6 Donec ac tincidunt urna.
7 Proin pretium, metus non porttitor lobortis, tortor sem rhoncus urna
8 quis finibus leo lorem sed lacus.
1 Lorem ipsum dolor sit amet, 

Expected result:

1 Lorem ipsum dolor sit amet, 
2 consectetur adipiscing elit  
3 Mauris porttitor enim sed tincidunt interdum. 
4 Morbi elementum erat nec nulla auctor, eget porta odio aliquet. 
5 Nam aliquet velit vel elementum tristique. 
6 Donec ac tincidunt urna. 
7 Proin pretium, metus non porttitor lobortis, tortor sem rhoncus urna 
8 quis finibus leo lorem sed lacus. 
1 Lorem ipsum dolor sit amet, 

UPDATE: I forgot an important detail in the original challenge: The same line can come multiple times, but never two times in a row so just deduplicating is not enough.
Japt -R, 11 bytes
This passes the test case but the spec still isn't clear enough
·ò¨ mÌò¦ mg

Try it