I don’t understand how to avoid concurrency when there are more sessions.
Let’s imagine there are some table User:

class User(Base):
    __tablename__ = "user"
    id: Mapped[UUID] = mapped_column(primary_key=True)
    email: Mapped[str] = mapped_column(unique=True)
    hashed_password: Mapped[str]

crud functions:

async def create(db: AsyncSession, user_scheme: UserCreate) -> User:
    user = User(
        **user_scheme.model_dump(exclude={"password"}),
        hashed_password=hash_password(user_scheme.password),
    )
    db.add(user)
    await db.commit()
    return user


async def exists_by_email(db: AsyncSession, user_email: str) -> bool:
    stmt = sql.select(sql.exists(User).where(User.email == user_email))
    res = await db.scalar(stmt)
    return bool(res)

and route function:

@router.post("/", status_code=201, response_model=UserRead)
async def register_user(db: SessionDepends, user_scheme: UserCreate) -> User:
    # step 1: check if is email taken
    if await crud.user.exists_by_email(db, user_scheme.email):
        raise HTTPException(409, "Email is already taken")
    
    # step 2: create and return user
    return await crud.user.create(db, user_scheme)

What if there are two clients that trying to register user with a same email and occur situations like below:

I want to learn how to properly write code in these cases?