Put this below code in `List A` event receiver. This method will copy only current item to List B that you add in the List A.

**Code:**

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

        using (SPSite site = new SPSite("http://MyWeb/sites/mysite/"))
        {
            using (SPWeb web = site.OpenWeb())
            {                
				SPList desList = web.Lists.TryGetList("B");
				if (desList != null)
				{
					SPListItem desItem = desList.Items.Add();
					foreach (SPField field in properties.item.Fields)
					{
						if (!field.ReadOnlyField && !field.Hidden && field.InternalName != "Attachments")
						{
							if (desItem.Fields.ContainsField(field.InternalName))
							{
								desItem[field.InternalName] = properties.item[field.InternalName];

							}

						}
					}
					desItem.Update();
				}
            }
        }

    }

This method will copy the list item from `List A` to `List B` after you add list item to List A.

Hope this will help you!