// Plays an animation only if we are not playing it already.
function OnMouseEnter() {
    if (!animation.isPlaying)
        animation.Play();
}

using UnityEngine;
using System.Collections;

public class example : MonoBehaviour {
    void OnMouseEnter() {
        if (!animation.isPlaying)
            animation.Play();
        
    }
}

import UnityEngine
import System.Collections

class example(MonoBehaviour):

    def OnMouseEnter():
        if not animation.isPlaying:
            animation.Play()