If I do wudhu for the Fajr prayer and don't break it then for Zuhr do another (fresh) wudhu is this rewarded (as wudhu washes away minor sins) or considered sinful due to wastage of water?
The following Hadith show that it is indeed rewarding to do wudhu whilst already in the state of wudhu:
The Prophet said: He who performs ablution whilst already in the state of purity is given ten good merits.
Kanz al-Ummal, no. 26042
Imam al-Sadiq said: He who renews his ablution without an impurity, Allah renews his repentance without him asking for forgiveness.
Wasael al-Shi'aah, v.1, p.264, no.7
Imam al-Sadiq said: Ablution upon ablution is light upon light.
Ibid. p.265, no.8