I don't think there is a problem with taking Cymbalta and eating grapefruit.
Read the medication leaflet that you received with the medication.
You can always ask your pharmacist as well.
I am not a fan of Cymbalta.
That being said, grapefruit contains a substance that increases the half-life of many medications. Thus, ingesting grapefruit is equivelant to increasing the dosage. That is why "drinking grapefuit juice is prohibited" is on the bottle of many prescriptions.
How much effective a single serving of a half-grapefruit will be in increasing the effect of a given drug is open to conjecture.