The text of Great Diplomat just references Leadership for the rules on what kind of cohort you get
You get two cohorts, one from each ability. Great Diplomat gives you a cohort, and Leadership gives you a cohort, and neither ability counts as the other or anything like that, so you can make use of both. Note that the +2 Leadership from Great Diplomat applies to the regular Leadership feat as well.
The following is all the text of the Leadership feat:
Character level 6th.
When taking Great Diplomat the Prerequisite doesn't matter, because Great Diplomat bypasses them. When taking Leadership, you must follow it like normal.
Having this feat enables the character to attract loyal companions and devoted followers, subordinates who assist her. See the table below for what sort of cohort and how many followers the character can recruit.
This is fluff, basically.
Several factors can affect a character’s Leadership score, causing it to vary from the base score (character level + Cha modifier). A character’s reputation (from the point of view of the cohort or follower he is trying to attract) raises or lowers his Leadership score:
Your leadership score, which is a stat you have independent of either feat, is total level+Cha mod+modifiers
Great Diplomat is likely to fetch you the 'great renown' modifier, based on its fluff, in addition to its automatic +2 leadership.
Other modifiers may apply when the character tries to attract a cohort:
Of note is the fact that having a cohort does not reduce your leadership for additional cohorts.
Followers have different priorities from cohorts. When the character tries to attract a new follower, use any of the following modifiers that apply.
This is irrelevant for Great Diplomat, and pretty self explanatory for Leadership.
A character’s base Leadership score equals his level plus any Charisma modifier. In order to take into account negative Charisma modifiers, this table allows for very low Leadership scores, but the character must still be 6th level or higher in order to gain the Leadership feat. Outside factors can affect a character’s Leadership score, as detailed above.
The character can attract a cohort of up to this level. Regardless of a character’s Leadership score, he can only recruit a cohort who is two or more levels lower than himself. The cohort should be equipped with gear appropriate for its level. A character can try to attract a cohort of a particular race, class, and alignment. The cohort’s alignment may not be opposed to the leader’s alignment on either the law-vs-chaos or good-vs-evil axis, and the leader takes a Leadership penalty if he recruits a cohort of an alignment different from his own.
Cohorts earn XP as follows:
The cohort does not count as a party member when determining the party’s XP.
Divide the cohort’s level by the level of the PC with whom he or she is associated (the character with the Leadership feat who attracted the cohort).
Multiply this result by the total XP awarded to the PC and add that number of experience points to the cohort’s total. If a cohort gains enough XP to bring it to a level one lower than the associated PC’s character level, the cohort does not gain the new level—its new XP total is 1 less than the amount needed attain the next level.
These two are the suites of rules which constitute most of what Great Diplomat needs to reference.
Number of Followers by Level:
The character can lead up to the indicated number of characters of each level. Followers are similar to cohorts, except they’re generally low-level NPCs. Because they’re generally five or more levels behind the character they follow, they’re rarely effective in combat.
Followers don’t earn experience and thus don’t gain levels. However, when a character with Leadership attains a new level, the player consults the table above to determine if she has acquired more followers, some of which may be higher level than the existing followers. (You don’t consult the table to see if your cohort gains levels, however, because cohorts earn experience on their own.)
The important thing here is "You don’t consult the table to see if your cohort gains levels, however, because cohorts earn experience on their own."
Also self explanatory. The cohort level, of course, is just the maximum level the cohort starts at; it may progress up to your level-2 via XP, and so it might be higher or lower than the table indicates for your first cohort when you gain your second.
and that's it. No prohibitions against gaining additional copies of the Leadership feat's effects, no special interactions, nothing like that. The only thing of particular note is that Great Diplomat will be effected by Epic Leadership under the popular interpretation that the latter alters Leadership rather than supplying an additional contingent of allies.