should i change my dress if i attend a wedding before mine & that bride is wearing the same dress??

These instances can occur with ready made wedding gowns. It is ultimately the brides choice as to whether or not she wants to wear the same dress or change this. It is of course, a difficult decision to make and one of the key drivers behind the brides decision will be the cost associated. 
If you custom make your wedding gown you eliminate these instances as the custom made gown will be original and of a personal style made specific for the bride in the style, colour, fabric and detail of her choice.

Great question. Personally I would subtly change it by adding more lace &/or extra train length.

Even add more accessories for example a beautiful nice crown which you can read an article we wrote a few weeks back;

Adding long gloves on a detachable overskirt depending on the style you have chosen.

Bear in mind every bride looks different in the same dress ~ you might be able to wear it better than other bride.

Hope your day is awesome and enjoy it!!!