Investigate if only single lockup exists per owner+duration+denom pair

[mattverse]

Background

The current lockup module "assumes" that we have a single lockup in AddToExistingLock(which is when we're trying to add coins to existing locks) and add coins to the first lock out of the given array.

func (k Keeper) AddToExistingLock(ctx sdk.Context, owner sdk.AccAddress, coin sdk.Coin, duration time.Duration) ([]types.PeriodLock, error) {
	locks := k.GetAccountLockedDurationNotUnlockingOnly(ctx, owner, coin.Denom, duration)
	// if existing lock with same duration and denom exists, just add there
	if len(locks) > 0 {
		lock := locks[0]
		_, err := k.AddTokensToLockByID(ctx, lock.ID, owner, coin)
		if err != nil {
			return nil, sdkerrors.Wrap(sdkerrors.ErrInvalidRequest, err.Error())
		}
	}
	return locks, nil
}

We want to ensure this is a safe, by changing len(locks) = 1 from the current simple check len(locks) > 0.

In order to make this change, we first need to make sure that only a single lockup exists per owner + duration + denom pair, and then change this logical gate to check that len(locks) = 1

cref: #1979 (comment)

Acceptance Criteria

• Investigate and make sure that only a single lockup exists per owner + duration + denom pair in the current state, make sure there's no edge cases in being able to make multiple locks per owner + duration + denom pair
• Change checks in AddToExistingLock to be len(locks) = 1 from len(locks) > 0