You have a Salesforce Custom Object that has a relationship to multiple objects, which shows up twice in Marketo with the same name. When trying to enable the second object in Marketo, you seeing an error like this: "An object with the same name is currently enabled for sync. Please give the object a unique name and retry."
Marketo requires that each custom object have a unique label. You will see this error when the labels in Marketo are the same for both custom objects. In order to enable it, you'll need to change the label in Marketo for the second custom object that is attempting to be synced. The easiest thing to do is append a letter in parenthesis to denote which object it is for (e.g., "Example Object (A)" for the Account).
This will allow the custom object to sync into Marketo and the API information in the back end will remain unchanged, allowing the records and changes to sync from Salesforce into Marketo.