How to query the list using CAML Query to List's multi allow user field?

I have a list in which there is one column which is of type People or Group with multi allow. I have created Item Adding event receiver in which I am checking the entered people value is already available or not. If it is then I am displaying the error message.

To query the People or Group field I found this solution, I tried it in my code which is as follows:

public override void ItemAdding(SPItemEventProperties properties)
{
    base.ItemAdding(properties);

    SPQuery query = null;            
    SPList list = properties.List;

    int i = 1;

    string strUsers = (string)properties.AfterProperties[FieldNames.Manager];

    string strQuery = "<Where> <Or>";

    SPFieldUserValueCollection newUsers = new SPFieldUserValueCollection(properties.Web, strUsers);

    foreach (SPFieldUserValue value in newUsers)
    {
        SPUser user = null;
        if (value.LookupId > 0)
        {
            user = value.User;
        }
        else if (value.LookupId < 0)
        {
            user = properties.Web.EnsureUser(value.LookupValue);
        }
        if (i % 2 != 0)
        {
            strQuery += "<Or>";
        }

        strQuery += "<Includes> <FieldRef Name='" + FieldNames.Manager + "' LookupId='TRUE'/> <Value Type='Integer'>" + user.ID + "</Value></Includes>";

        if (i % 2 != 0)
        {
            strQuery += "</Or>";
        }

        i++;
    }
    strQuery += "</Or></Where>";

    query = new SPQuery();
    query.Query = strQuery;

    SPListItemCollection coll = list.GetItems(query); // I am getting error here..

    if(coll.Count > 0)
    {
        properties.ErrorMessage = "Manager is already assigned to other department";
        properties.Status = SPEventReceiverStatus.CancelWithError;
    }
}

The resultant query generated by my program is:

<Query>
<Where> 
    <Or>
        <Or>
            <Includes> 
                <FieldRef Name='oiplbManager' LookupId='TRUE'/> <Value Type='Integer'>1</Value>
            </Includes>
        </Or>
            <Includes> <FieldRef Name='oiplbManager' LookupId='TRUE'/> <Value Type='Integer'>18</Value>
            </Includes>
        </Or>
</Where>
</Query>

What am I missing?