If had general anaesthetic before does this mean i can safely have it again without allergic reactions?

Yes: Many patients have multiple / serial general anesthetics - sometimes every few days. Examples include management of a burn injury, or multiple fractures / trauma from a car accident. It is possible to have an allergic reaction on repeat exposure to a drug. Typically this occurs with antibiotics, not anesthesia medications.

Anesthetic Allergy: A reaction to general anesthesia can be one of several different medical conditions. True allergies, such as the kind most people talk about, a reaction related to a genetic abnormalities or an ongoing diseases that limit your tolerance to anesthesia. If neither you nor your blood relatives have had a reaction before, you should be safe. If you are very ill the type and amount may be limited.