Dublicate order numbers

This happens once in a while: The order number generated from the plugin are dublicated. It′s the free plugin.

What could be the cause – and how do we fix it?

See attached.
https://dl.dropboxusercontent.com/u/84173534/woo-ordernumbers.png

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### WordPress Environment ###

Home URL: https://therabands.se
Site URL: https://therabands.se
WC Version: 3.0.5
Log Directory Writable: ?
WP Version: 4.7.5
WP Multisite: –
WP Memory Limit: ? 40 MB - Vi rekommenderar att du anv?nder minst 64MB i minne. L?s: ?ka minnet som allokeras till PHP
WP Debug Mode: –
WP Cron: ?
Language: sv_SE

### Server Environment ###

Server Info: Apache
PHP Version: 7.0.18-0ubuntu0.16.04.1
PHP Post Max Size: 100 MB
PHP Time Limit: 0
PHP Max Input Vars: 1000
cURL Version: 7.47.0
OpenSSL/1.0.2g

SUHOSIN Installed: –
MySQL Version: 5.7.18
Max Upload Size: 50 MB
Default Timezone is UTC: ?
fsockopen/cURL: ?
SoapClient: ?
DOMDocument: ?
GZip: ?
Multibyte String: ?
Remote Post: ?
Remote Get: ?

### Database ###

WC Database Version: 3.0.5
WC Database Prefix: wp_
woocommerce_sessions: ?
woocommerce_api_keys: ?
woocommerce_attribute_taxonomies: ?
woocommerce_downloadable_product_permissions: ?
woocommerce_order_items: ?
woocommerce_order_itemmeta: ?
woocommerce_tax_rates: ?
woocommerce_tax_rate_locations: ?
woocommerce_shipping_zones: ?
woocommerce_shipping_zone_locations: ?
woocommerce_shipping_zone_methods: ?
woocommerce_payment_tokens: ?
woocommerce_payment_tokenmeta: ?
MaxMind GeoIP Database: ?

### Security ###

Secure connection (HTTPS): ?
Hide errors from visitors: ?

### Active Plugins (23) ###

Autoptimize: av Frank Goossens (futtta) – 2.1.2
GetSiteControl Widgets: av getsitecontrol – 2.0.4
Kraken Image Optimizer: av Karim Salman – 2.6.2
MailChimp for WooCommerce: av MailChimp – 2.0.0
PixelYourSite PRO: av PixelYourSite – 6.1.3
Regenerate Thumbnails: av Alex Mills (Viper007Bond) – 2.2.6
Storefront Powerpack: av WooThemes – 1.3.1 – 1.4.1 finns tillg?nglig
WooCommerce Dynamic Pricing & Discounts: av RightPress – 1.1.2
Woo Custom and Sequential Order Number Lite: av VJInfotech – 1.1
WooCommerce Advanced Free Shipping: av Jeroen Sormani – 1.1.1
Advanced Messages for WooCommerce: av Jeroen Sormani – 1.1.1
WooCommerce e-conomic Integration: av wooconomics – 1.9.25
WooCommerce EU VAT Number: av WooCommerce – 2.2.0 – 2.3.0 finns tillg?nglig
WooCommerce Klarna Gateway: av WooCommerce – 2.3.6 – 2.3.11 finns tillg?nglig
WooCommerce Stripe Gateway: av WooCommerce – 3.1.8
WooCommerce AdWords Conversion Tracking: av Wolf+B?r GmbH – 1.3.5
WooCommerce Google Analytics Pro: av SkyVerge – 1.3.3 – 1.3.4 finns tillg?nglig
WooCommerce AdWords Dynamic Remarketing: av Wolf + B?r GmbH – 1.3.3
WooCommerce: av Automattic – 3.0.5 – 3.0.9 finns tillg?nglig
Yoast SEO Premium: av Team Yoast – 4.7.1
WooCommerce Product Feed Manager: av Michel Jongbloed – 1.7.4
Yoast SEO: Local: av Team Yoast and Arjan Snaterse – 4.7.1
Yoast SEO: WooCommerce: av Team Yoast – 4.7

### Settings ###

API Enabled: ?
Force SSL: ?
Currency: SEK (SEK)
Currency Position: right_space
Thousand Separator: 
Decimal Separator: ,
Number of Decimals: 2
Taxonomies: Product Types: external (external)
grouped (grouped)
simple (simple)
variable (variable)

Taxonomies: Product Visibility: exclude-from-catalog (exclude-from-catalog)
exclude-from-search (exclude-from-search)
featured (featured)
outofstock (outofstock)
rated-1 (rated-1)
rated-2 (rated-2)
rated-3 (rated-3)
rated-4 (rated-4)
rated-5 (rated-5)

### WC Pages ###

Butikbas: #4 - /shop/
Varukorg: #5 - /cart/
Kassan: #6 - /cart/checkout/
Mitt konto: #7 - /my-account/

### Theme ###

Name: therabands theme
Version: 1.0
Author URL: 
Child Theme: ?
Parent Theme Name: Storefront
Parent Theme Version: 2.1.8 – 2.2.3 finns tillg?nglig
Parent Theme Author URL: https://woocommerce.com/
WooCommerce Support: ?

### Templates ###

Overrides: theraband-shop/woocommerce/cart/proceed-to-checkout-button.php
theraband-shop/woocommerce/loop/sale-flash.php
theraband-shop/woocommerce/single-product/product-image.php
theraband-shop/woocommerce/single-product/sale-flash.php

### Klarna ###

Test mode: Enabled Your Klarna Checkout account must be in test mode for all configured countries in order for it to work
please get in touch with Klarna to check your account status

Sweden: ?

Villkorssida: ?