At first level, peace clerics gain heroism and sanctuary. But don't all clerics already have access to all cleric spells (at the appropriate levels), which includes sanctuary? Only wizards, warlock, sorcerers and rangers have access to a limited number of spells.
Domain Spells are always prepared and don’t count against your total prepared spells.
The rules for Domain Spells state:
Each domain has a list of spells — its domain spells — that you gain at the cleric levels noted in the domain description. Once you gain a domain spell, you always have it prepared, and it doesn’t count against the number of spells you can prepare each day.
So yes, sanctuary is already on the cleric spell list, but a Peace cleric has it prepared without it counting against their total prepared spells since it is one of their domain spells. Other clerics have to prepare sanctuary as usual, Peace clerics do not.
This phenomenon is not unique to the Peace domain. The chief offender is the Life domain: every single one of their domain spells is already a cleric spell. So the Life domain doesn't get any new spells at all.