This morning I logged in to send a friendly player a tech (Scanning 4). Confirmed, then switched to the message screen to send confirmation. Switched to my other game.
Came back to the game (this one) later, and found event notices that both Scanning 4 and Scanning 5 had been sent. In addition, my message was sent to him twice. (The time on when Scanning 5 was sent would have been while I was composing my message.)
The sequence of actions was (this was on my phone):
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enter game, find other player’s star, click on it and “View”
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click the “eye” icon under the avatar image, then scroll down to the trading drop-down
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“Scanning 4” is already selected, hit “Share Technology” and confirm.
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scroll back down to confirm it went, and to check out which level 5 techs he has to ask for
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Note: I did not scroll down far enough to view the “Share Technology” drop-down again
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Switch to diplomacy, open the existing thread and reply to it.
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Main Menu, “My Games” → other game
I’ve had something similar happen on my old desktop. It doesn’t happen on my newer faster desktop. Both running firefox.
Workarounds
Desktop: Click buttons slowly. Do not release mouse button until UI-button has depressed.
Mobile: Click quickly maybe? Wait a few seconds, if the event appears pressed down do not click a second time. Reload the page and retry.
Below directed to @JayKyburz
I believe in my case it is connected to the mouseup event sometimes not triggering. If you happen to trigger down, down, up on a button it will trigger the event twice! I’ve tried to locate the specific bug location but can not figure it out. This happens with all UI-buttons on my old computer but not all the time.
The OP’s mobile case may be caused by interpreting a single long click as multiple clicks. I’m not sure how the mobile stuff works.
hrm, so are with thinking that the “OK” button on the confirm dialog triggered twice, sending two “share tech” events?
Sounds like there is two bugs. One around double clicking buttons, but also when sharing tech it should be “share level x” not “share a level” so that a duplicated event would fail.
Double clicking can be hazardous on mobile.
I managed to buy 5 Science instead of 1 in a recent game. It was day 4 I think so it sort of crippled my Economic growth 
That makes complete sense. I tried a couple of times to duplicate it, but the UI was completely responsive, and all I did was share some tech with an AFK player. Then I ran out of techs I had 2 levels advantage in, so I had to stop. Plus it was costing me money. 
Thinking back, I had just restarted the phone, so it may have been slow as stuff was still starting up in the background (plus processing notifications, downloading mail, etc.).
Definitely “share level 4” would fix half the problem, since it can’t happen twice.
As to the other half of the problem, I guess I should just learn to be a bit less impatient. (I’ve built some HTML/Javascript UI components, and I know that it’s impossible to guarantee complete responsiveness in all circumstances.)
Like the other responder, I’ve also accidentally made multiple purchases because the UI lags. (You’d think I might have learned.) It’s a general problem of any computer UI, because there’s no true tactile sense that yes, the button got pressed. It’s just worse on phones, because sometimes you simply can’t tell the difference between missing the button or the UI not responding to the button press. (Perhaps the phone should respond to touch events that aren’t otherwise handled to show where you (me) poked your (my) fat finger, that wasn’t actually on any active element.)
