Panic when adding duplicate files

When running `target/debug/logtail ./foo.txt ./foo.txt`:
```
Loading...
./foo.txt
./foo.txt done.
./foo.txt
./foo.txt done.
thread 'main' panicked at 'index out of bounds: the len is 1 but the index is 1', src/bin/../custom/ui.rs:57:45
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace
```

Worth noting that `linemux` takes the approach of [ignoring duplicates](https://github.com/jmagnuson/linemux/blob/21cfee16fe4c8c5029df8f39066b1a2240898c2e/src/reader.rs#L182-L184) and only tailing a single instance of the file. I should make a note of that in the docs.