In the beginning of this week's פרשה we are told (א:י"א) that בני ישראל built two large cities for פתם - פרעה and רַעַמְסֵס. This may sound rather similar to the city in which יוסף placed his family (בראשית מ"ז:י"א). However, אבן עזרא points out that in fact the city mentioned in ויגש is different - רַעְמְסֵס - actually pronounced Ra-me-seis as opposed to the one in our פרשה, pronounced Ra-am-seis. Thus, they are not the same city. A בעל קריאה, therefore, should probably be careful to distinguish between the two.
Surprisingly, though, רש"י explains that בני ישראל made these already existing cities into storage cities for פרעה. It seems from רש"י's words that he assumed that the two cities were the same and that is why he explained the apparent difficulty of building a city that already exists. This is hard to understand not only because of what אבן עזרא stated but also, why would פרעה place his storage in the city in which בני ישראל dwelled.