Just remove the superfluous partition via the disk utility. You should be able to then extend your Macintosh HD to use up the free space as well.
Thats what I thought. I select the partition. The "-" character is greyed out. As in, it cant be clicked on. I assume thats how you remove it... no?
Try booting into the recovery partition and doing it.
Under the Partition tab, click on the partition you accidentally created. Then click the '-' button.
After that, drag the slider of the original partition back to the bottom to recover space.
diskutil list
/dev/disk0
#: TYPE NAME SIZE IDENTIFIER
0: GUID_partition_scheme *251.0 GB disk0
1: EFI EFI 209.7 MB disk0s1
2: Apple_CoreStorage 125.4 GB disk0s2
3: Apple_Boot Recovery HD 650.0 MB disk0s3
/dev/disk1
#: TYPE NAME SIZE IDENTIFIER
0: Apple_HFS Macintosh HD *125.1 GB disk1 <---- I believe, this is the second partition I accidentally created, which I want to remove
Logical Volume on disk0s2
8F5A679C-06AB-44CC-BD46-33E327446D80
Unencrypted
Code:diskutil list /dev/disk0 #: TYPE NAME SIZE IDENTIFIER 0: GUID_partition_scheme *251.0 GB disk0 1: EFI EFI 209.7 MB disk0s1 2: Apple_CoreStorage 125.4 GB disk0s2 3: Apple_Boot Recovery HD 650.0 MB disk0s3 /dev/disk1 #: TYPE NAME SIZE IDENTIFIER 0: Apple_HFS Macintosh HD *125.1 GB disk1 <---- I believe, this is the second partition I accidentally created, which I want to remove Logical Volume on disk0s2 8F5A679C-06AB-44CC-BD46-33E327446D80 Unencrypted
diskutil cs revert /